Add test to verify the accuracy of multisample scaled blit using a shader program

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 extension
   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 don't produce expected
results on other hardware. So, this might not land in upstream piglit.
Test passes with all of the scaling factors between 0.1 to 2.5 on
Intel's i965 drivers. Patches for scaled blitting support on i965 are
out for review on mesa-dev mailing list.

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>
Add test to verify the visual quality of multisample scaled blitting

This test is developed to visually compare the output images by:
1. Scaled blit using ext_framebuffer_multisample_blit_scaled.
2. Scaled blit using 2 glBlitFramebuffer() calls.
   Multisample buffer resolve followed by scaled blit using
   GL_LINEAR filter.

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