api: check for window focus when accepting input (#317)

## Link to GitHub Issue, if one exists
Fixes #139 

## Description of change
Add an option for detecting window focus when accepting input.

The new default will be to have the following:
 * naive bindings will check for window focus
* rawinput binds are not affected, will continue to be accepted
regardless of focus

Other options (always / never choices) can be picked by the user to
tweak the behavior if needed.

Clean up usage of `0xff` constant to represent invalid naive vkey.

## Testing
Still in progress, especially in regards to performance.

On a i7-9700 system, 10000 calls to `superexit::has_focus()` takes
250-350 microseconds, so this should not be a big deal at all.

src/spice2x/launcher/options.cpp

@@ -2170,6 +2170,23 @@ static const std::vector<OptionDefinition> OPTION_DEFINITIONS = {
         .setting_name = "20",
         .category = "I/O Options",
     },
+    {
+        // InputRequiresFocus
+        .title = "Input Requires Focus",
+        .name = "inputfocus",
+        .desc =
+            "Determine input behavior when game window is not in focus.\n\n"
+            "never: allow input regardless of focus\n"
+            "naive (default): only naive bindings check for window focus, rawinput binds are always allowed\n"
+            "always: only process input when window is in focus",
+        .type = OptionType::Enum,
+        .category = "I/O Options",
+        .elements = {
+            {"never", ""},
+            {"naive", ""},
+            {"always", ""},
+        },
+    },
     {
         // NostalgiaPoke
         .title = "Nost Screen Poke",