Added SDL_ALPHA_OPAQUE_FLOAT and SDL_ALPHA_TRANSPARENT_FLOAT

Fixes https://github.com/libsdl-org/SDL/issues/10684

include/SDL3/SDL_pixels.h

@@ -47,6 +47,15 @@ extern "C" {
  */
 #define SDL_ALPHA_OPAQUE 255
 
+/**
+ * A fully opaque floating point alpha value.
+ *
+ * \since This macro is available since SDL 3.0.0.
+ *
+ * \sa SDL_ALPHA_TRANSPARENT_FLOAT
+ */
+#define SDL_ALPHA_OPAQUE_FLOAT 1.0f
+
 /**
  * A fully transparent 8-bit alpha value.
  *
@@ -56,6 +65,15 @@ extern "C" {
  */
 #define SDL_ALPHA_TRANSPARENT 0
 
+/**
+ * A fully transparent floating point alpha value.
+ *
+ * \since This macro is available since SDL 3.0.0.
+ *
+ * \sa SDL_ALPHA_OPAQUE_FLOAT
+ */
+#define SDL_ALPHA_TRANSPARENT_FLOAT 0.0f
+
 /** Pixel type. */
 typedef enum SDL_PixelType
 {