Add test to verify the accuracy of multisample scaled blitting

This test verifies the accuracy of scaled blitting on Intel's i965
drivers by comparing the output images by:
1. multisample buffer to single sample buffer scaled blit using
   EXT_multisample_framebuffer_blit_scaled extension.
2. multisample buffer to single sample buffer scaled blit implemented
   using glsl shader program.

Note: This test is specific to Intel's implementation of extension
EXT_multisample_framebuffer_blit_scaled and may not produce expected
results on other hardware. Test passes with all of the scaling factors
between 0.1 to 2.5 on Intel's i965 drivers and NVIDIA's proprietary
linux drivers.

V2: Add testing for scaled blitting using multisample framebuffer with
    renderbuffer attachment. This helps verify that mcs_fetch() is
    happening correctly for each sample.

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Add test to verify the error conditions in ext_multisample_framebuffer_blit_scaled

Note: Test passes on Intel's i965 drivers. Patches for scaled
blitting support on i965 are out for review on mesa-dev mailing list.

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>