renderer/native: Allow manual selection of the framebuffer format
Allow manual selection of the framebuffer format using the environment
variable MUTTER_FORCE_FRAMEBUFFER_FORMAT
with a FourCC string.
Examples that work with Intel graphics:
XR24 = 24-bit "True color", 4 bytes per pixel (mutter's default)
XR30 = 30-bit "Deep color", 4 bytes per pixel
RG16 = 16-bit "High color", 2 bytes per pixel