mesa: replace gl_framebuffer::_IntegerColor wih _IntegerBuffers

mesa: replace gl_framebuffer::_IntegerColor wih _IntegerBuffers

Use a bitmask to indicate which color buffers are integer-valued, rather
than a bool.  Also, the old field was mis-computed.  If an integer buffer
was followed by a non-integer buffer, the _IntegerColor field was wrongly
set to false.

This fixes the new piglit gl-3.1-mixed-int-float-fbo test.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>