hi,

You use alpha with OpenGL renderer... no chance... it's a big bug of SDL2 now....

in SDL2 readme :

KNOW BUGS:
*************

- SDL_CreateRenderer with SDL_RENDERER_ACCELERATED is broken if you use SDL_BLENDMODE or
tranparency color with OpenGL renderer, you need to use SDL_RENDERER_SOFTWARE.
Dont ask me to fix that, i dont have any information on it, maybe fix on next release of MorphOS (3.16)..)