OpenGL1: Overbright without gamma ramps

[zturtleman]

Fix the game being too dark when gamma ramps are not supported (on some GPUs or displays and in windowed mode) or disabled using r_ignorehwgamma 1. (SDL 3 is going to remove window gamma ramps altogether.) A simple option (compatible with all OpenGL versions) is drawing a white texture (1,1,1,1) with blend mode srcFactor=GL_DST_COLOR dstFactor=GL_ONE to effectively double the destination pixel values like done with overbright gamma ramps. Though I'm not sure about easy ways to supporting r_intensity and r_gamma.