1. You can test with ScummVM 2.5.1, put OpenGL in Graphics mode (Graphics tab in Options)
Press ALT-S directly on first screen, png file created.

2. You can test with GRIM games, just launch game in OpenGL, and you can see texture glish : face of hero (not present with MacMini)

3. With PRBoom 2.6.1 : Options / General and choose video mode OpenGL
Just note PrBoom freeze on MacMini (R200) and not with R300..
If PrBoom working, you can see transition screen glish and sky glish on game.

Last little question, have you plan to check GL_BGRA blend mode ?
(it's most important bug in SDL2 opengl renderer)

Thanks