When using EGL the mesa+nvidia stack are unable to offload 32bit
framebuffers despite having this capability on GLX. In practice the X11
server does not support alpha windows so we dont need the alpha
component in our framebuffer. We previously had alpha specified in our
framebuffer since we do alpha texturing but testing shows this isnt
required for mesa/intel or nvidia drivers and we must pick a 24bit
config for users to enable render offloading for mixed gpu systems.
fixes#6984
674bd3f821