overlay: toast notifications (#704)

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

## Description of change
Adds toast notifications to the overlay.

Toast notifications are transient windows (shown for about 4 seconds).
These are drawn on top of the game even when the overlay is inactive
(i.e., when no window is open), and does not cause any input sink
behavior.

Add toasts for some common user-driven actions that could use a
notification, such as:

* screenshot
* card insert
* PIN macro
* API client connect/disconnect
* virtual printer
* screen resize toggle / scene switch

Add an option to change where the toasts are displayed, or to turn it
off entirely.

Add SDK function for it, and add it to the samples.

## Testing
Tested DDR (shown at bottom right, as is for most games) and RB (shown
top right by default).

src/spice2x/overlay/overlay.h

@@ -141,6 +141,10 @@ namespace overlay {
         bool hotkey_toggle = false;
         bool hotkey_toggle_last = false;
 
+        // true between new_frame()'s ImGui::NewFrame() and render()'s ImGui::Render(),
+        // so render() never runs without a matching NewFrame.
+        bool has_pending_frame = false;
+
         void init();
         void add_font(const char* font, ImFontConfig* config, const ImWchar* glyphs);
     };