rb: touch debug overlay, tweak scaling (#801)

## Link to GitHub Issue or related Pull Request, if one exists
n/a

## Description of change
Similar to #797, draw an overlay to debug touch issues for reflec beat.

For rb, we draw on the ImGui surface. Draw horizontal and vertical line
for triggered sensors, and also a bounding box if scaling is in use.

Make small tweaks to how scaling works.

## Testing
Verified volzza 2 / reflesia / 1st game (landscape KBR)

src/spice2x/games/rb/touch_debug.cpp

@@ -0,0 +1,139 @@
+#include "touch_debug.h"
+
+#include <array>
+#include <atomic>
+#include <cstring>
+#include <mutex>
+
+#include "external/imgui/imgui.h"
+#include "games/rb/rb.h"
+#include "games/rb/touch_defs.h"
+
+namespace games::rb {
+
+    struct TouchDebugState {
+        std::array<unsigned char, TOUCH_PACKET_SIZE> packet {};
+        bool is_landscape = false;
+    };
+
+    std::atomic_bool TOUCH_DEBUG_OVERLAY = false;
+    static std::atomic_bool TOUCH_ACTIVE = false;
+    static std::mutex TOUCH_DEBUG_STATE_M;
+    static TouchDebugState TOUCH_DEBUG_STATE;
+
+    static float touch_scale_factor() {
+        return TOUCH_SCALING / (float) TOUCH_SCALE_DEFAULT;
+    }
+
+    static void clear_touch_debug_state() {
+        std::lock_guard<std::mutex> lock(TOUCH_DEBUG_STATE_M);
+        TOUCH_DEBUG_STATE = {};
+    }
+
+    static TouchDebugState get_touch_debug_state() {
+        std::lock_guard<std::mutex> lock(TOUCH_DEBUG_STATE_M);
+        return TOUCH_DEBUG_STATE;
+    }
+
+    static bool packet_bit_active(
+            const std::array<unsigned char, TOUCH_PACKET_SIZE> &packet, int bit) {
+        return (packet[TOUCH_PACKET_DATA_OFFSET + bit / 8] & (1u << (bit % 8))) != 0;
+    }
+
+    static int sensor_center(int sensor, int sensor_count, int extent) {
+        return ((sensor * 2 + 1) * extent) / (sensor_count * 2);
+    }
+
+    bool touch_debug_overlay_enabled() {
+        return TOUCH_DEBUG_OVERLAY && TOUCH_ACTIVE.load(std::memory_order_acquire);
+    }
+
+    void touch_draw_debug_overlay() {
+        if (!touch_debug_overlay_enabled()) {
+            return;
+        }
+
+        const auto &io = ImGui::GetIO();
+        int width = static_cast<int>(io.DisplaySize.x);
+        int height = static_cast<int>(io.DisplaySize.y);
+        if (width <= 0 || height <= 0) {
+            return;
+        }
+
+        const float scale_factor = touch_scale_factor();
+        const float left = width * (1.f - scale_factor) / 2.f;
+        const float top = height * (1.f - scale_factor) / 2.f;
+        const float right = width - left;
+        const float bottom = height - top;
+
+        TouchDebugState state = get_touch_debug_state();
+        ImDrawList *draw_list = ImGui::GetBackgroundDrawList();
+        auto draw_line = [&](float x1, float y1, float x2, float y2) {
+            draw_list->AddLine(
+                ImVec2(x1, y1), ImVec2(x2, y2),
+                IM_COL32(0, 255, 64, 255), 2.f);
+        };
+
+        // show the valid input area when touch scaling restricts it
+        if (TOUCH_SCALING != TOUCH_SCALE_DEFAULT) {
+            draw_list->AddRect(
+                ImVec2(left, top), ImVec2(right, bottom),
+                IM_COL32(255, 255, 255, 255), 0.f, 0, 2.f);
+        }
+
+        // spread the usable X sensor centers 2..45 across the full touch area
+        for (int sensor = X_SENSOR_FIRST_ACTIVE; sensor <= X_SENSOR_LAST_ACTIVE; sensor++) {
+            if (!packet_bit_active(state.packet, X_SENSOR_FIRST_BIT + sensor)) {
+                continue;
+            }
+
+            int position = sensor_center(
+                sensor - X_SENSOR_FIRST_ACTIVE, X_SENSOR_ACTIVE_COUNT,
+                state.is_landscape ? height : width);
+            if (state.is_landscape) {
+                float y = top + position * scale_factor;
+                draw_line(left, y, right, y);
+            } else {
+                float x = left + position * scale_factor;
+                draw_line(x, top, x, bottom);
+            }
+        }
+
+        for (int sensor = 0; sensor < Y_SENSOR_COUNT; sensor++) {
+            if (!packet_bit_active(state.packet, Y_SENSOR_FIRST_BIT - sensor)) {
+                continue;
+            }
+
+            int position = sensor_center(
+                sensor, Y_SENSOR_COUNT,
+                state.is_landscape ? width : height);
+            if (state.is_landscape) {
+                float x = right - position * scale_factor;
+                draw_line(x, top, x, bottom);
+            } else {
+                float y = top + position * scale_factor;
+                draw_line(left, y, right, y);
+            }
+        }
+    }
+
+    void touch_debug_attach() {
+        clear_touch_debug_state();
+        TOUCH_ACTIVE.store(true, std::memory_order_release);
+    }
+
+    void touch_debug_detach() {
+        TOUCH_ACTIVE.store(false, std::memory_order_release);
+        clear_touch_debug_state();
+    }
+
+    void touch_debug_publish(const unsigned char *data, bool is_landscape) {
+        if (!TOUCH_DEBUG_OVERLAY) {
+            return;
+        }
+
+        std::lock_guard<std::mutex> lock(TOUCH_DEBUG_STATE_M);
+        memcpy(TOUCH_DEBUG_STATE.packet.data(), data, TOUCH_PACKET_SIZE);
+        TOUCH_DEBUG_STATE.is_landscape = is_landscape;
+    }
+}