gpu: Update comments above CHECK_DEVICE_MAGIC macro

src/gpu/SDL_gpu.c

@@ -21,7 +21,12 @@
 #include "SDL_internal.h"
 #include "SDL_sysgpu.h"
 
-// FIXME: This could probably use SDL_ObjectValid
+/* Normally this macro would use something like SDL_IsObjectValid, but in GPU's
+ * case we prioritize performance and be more trusting of application behavior
+ * than, say, SDL_Render, so trust that applications will be careful about
+ * disposing the device and its resources.
+ * -flibit
+ */
 #define CHECK_DEVICE_MAGIC(device, retval)  \
     CHECK_PARAM(device == NULL) {           \
         SDL_SetError("Invalid GPU device"); \