opengl: fix some things on GPUs with somewhat limited GLES support
Commit 1: correct the check for glBufferStorage support on OpenGL ES... it is not supported without an extension
Commit 2: handle missing support for blits on Wayland, by sampling from the screen texture
Commit 3: correct the support checks. The contrast one checked for framebuffer sizes, which isn't needed anymore, it did not check for blit support, which it relies on, and both can now work on Wayland without blit support