Mouse coordinates are floating point

You can get sub-pixel mouse coordinates and motion depending on the platform and display scaling.

Fixes https://github.com/libsdl-org/SDL/issues/2999

src/render/SDL_sysrender.h

@@ -249,10 +249,6 @@ struct SDL_Renderer
     /* List of triangle indices to draw rects */
     int rect_index_order[6];
 
-    /* Remainder from scaled relative motion */
-    float xrel;
-    float yrel;
-
     /* The list of textures */
     SDL_Texture *textures;
     SDL_Texture *target;