Make the functions in ext_framebuffer_multisample/common.cpp available globally

A lot of classes and functions in ext_framebuffer_multisample/common.cpp
can be utilized to develop new piglit test cases. This patch moves the
Fbo and test_pattern classes to a global location tests/util.
Functions from these classes will be utilized in my later patches for
tests in spec/arb_framebuffer_object.

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Add test to verify the accuracy of scaled blitting with GL_LINEAR filter

This test verifies the accuracy of scaled blitting from a single
sample buffer with GL_LINEAR filtering. It compares the output
from following rendering scenarios:
1. Scaled blit using a single sample framebuffer with texture /
   renderbuffer attachment.
2. Scaled blit using a glsl shader program.

V2: Move the test to spec/arb_framebuffer_object directory.

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Add test to verify scaled blitting of {texture, renderbuffer} attachments produce same output

This test verifies that same output is produced by scaled blitting
of texture or renderbuffer color attachments with GL_LINEAR filter.
It compares the output from following rendering scenarios:
1. Scaled blit using a single-sampled framebuffer with texture
attachment.
2. Scaled blit using a single-sampled framebuffer with renderbuffer
attachment.

V2: Move the test to spec/arb_framebuffer_object directory.

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