cmake: Adjust Enabled Backends list for render drivers to catch opengles2.

This happens to work here because we don't have anything too complex, afaik.
Rejecting strings with underscores was crucial for other things, like video
with all its X11 variables, so this can't be applied to the other subsystems.

(Also, renaming the `SDL_VIDEO_RENDER_OGL_ES2` variable might be nice, too,
but let's keep this simple for now.)

Fixes #13938.

cmake/macros.cmake

@@ -395,7 +395,7 @@ function(SDL_PrintSummary)
   if(SDL_VIDEO_DRIVER_X11)
     PrintEnabledBackends("X11 libraries" "^SDL_VIDEO_DRIVER_X11_([A-Z0-9]*)$")
   endif()
-  PrintEnabledBackends("Render drivers" "^SDL_VIDEO_RENDER_([A-Z0-9]*)$")
+  PrintEnabledBackends("Render drivers" "^SDL_VIDEO_RENDER_([A-Z0-9_]*)$")
   PrintEnabledBackends("GPU drivers" "^SDL_GPU_([A-Z0-9]*)$")
   PrintEnabledBackends("Audio drivers" "^SDL_AUDIO_DRIVER_([A-Z0-9]*)$")
   PrintEnabledBackends("Joystick drivers" "^SDL_JOYSTICK_([A-Z0-9]*)$")