video: Let video targets optionally decide their default OpenGL configs. This is necessary because the Raspberry Pi is a strange beast, that believes it has OpenGL support (through glX?) but generally has GLES2 support. So when using the raspberry video target, we need to force this to default to a GLES2 context, or by default SDL_CreateWindow() will fail, deep down when it tries to load the proper GL library. Fixes testsprite2 (and basically everything else that wasn't testgles2) when run on a Raspberry Pi without a X server. Please note that other targets might also need this filled in, the Raspberry Pi is just the most prominent and readily-available System-On-A-Chip style thing on my desk. :)