ReadPixels: Force ALPHA to 1 while rebasing RGBA values for GL_RGB format

ReadPixels: Force ALPHA to 1 while rebasing RGBA values for GL_RGB format

When performing a ReadPixels operation, we may be reading from a buffer that
stores alpha values, but that is actually representing a buffer with no alpha
channel. In this case, while rebasing the values, touch up all alpha values
read to 1.0.

This commit fixes the following piglit (sub) tests:

	ARB_texture_float/fbo-colormask-formats
		GL_RBG16F_ARB
	EXT_texture_snorm/fbo-colormask-formats
		GL_RGB16_SNORM
		GL_RGB8_SNORM
		GL_RGB_SNORM

It likely improves the results of other tests as well, but a PASS remains
elusive due to additional bugs.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>