mesa: fix the selection of soft renderbuffer color formats

mesa: fix the selection of soft renderbuffer color formats

This fixes a regression from the recent glReadPixels changes found
with the piglit hiz tests.

Use either MESA_FORMAT_RGBA8888 or MESA_FORMAT_RGBA8888_REV for color
buffers depending on endian-ness.  Before, the gl_renderbuffer::Format
field was MESA_FORMAT_RGBA8888 but the data was really stored as
MESA_FORMAT_RGBA8888_REV when using a little endian machine.

Getting this right matters now that we can access renderbuffer data
without going through the span functions (namely glReadPixels() +
MapRenderbuffer()).