mesa: Add support to allow blitting to multiple color draw buffers

mesa: Add support to allow blitting to multiple color draw buffers

This patch fixes a blitting case when drawAttachment->Texture ==
readAttachment->Texture. It was causing an assertion failure in
intel_miptree_attach_map() with gles3 conformance test case:
framebuffer_blit_functionality_minifying_blit

Number of changes in this file look scary. But most of them are caused
by introducing a big for loop to support rendering to multiple color
draw buffers.

V2: Fixed a case when number of draw buffer attachments are zero.
V3: Put a for loop in blit_nearest() and blit_linear() functions in to
    support blitting to multiple color draw buffers.
V4: Remove variable declaration in for loop to avoid MSVC compilation
    issues.

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>