intel: Add multisample scaled blitting in blorp engine

In traditional multisampled framebuffer rendering, color samples must be
explicitly resolved via BlitFramebuffer before doing the scaled blitting
of the framebuffer. So, scaled blitting of a multisample framebuffer
takes two separate calls to BlitFramebuffer.

This patch implements the functionality of doing multisampled scaled
resolve using just one BlitFramebuffer call. Important changes involved
in this patch are listed below:
    - Use float registers to scale and offset texture coordinates.
    - Change offset computation to consider float coordinates.
    - Round the scaled coordinates down to nearest integer.
    - Modify src texture coordinates clipping to account for scaling..
    - Linear filter is not yet implemented in blorp. So, don't use
      blorp engine to do single sampled scaled blitting.

Note: Observed no piglit regressions on sandybridge & ivybridge with
these changes.
i965: Enable ext_framebuffer_multisample_blit_scaled on intel h/w

This patch enables ext_framebuffer_multisample_blit_scaled extension
on intel h/w >= gen6.

Note: Patches for piglit tests to verify this functionality are out
for review on piglit mailing list. Tests pass for all of the scaling
factors from 0.1 to 2.4.

Comment from Paul Berry:
I have some concerns about the image quality of the method you've
implemented.  As I understand it, the primary use case of this extension
is to allow the client to do multisampled rendering at slightly less
than screen resolution (e.g. 720p instead of 1080p), and then blit the
result to the screen in one step while keeping most of the quality
benefits of multisampling.  Since your implementation is effectively
equivalent to downsampling and then blitting using GL_NEAREST filtering,
my fear is that it will lead to blocky artifacts that are severe enough
to negate the benefit of multisampling in the first place.

Before we turn this extension on in the Intel driver, I'd like to look
at a comparison of:

(1) your technique
(2) downsampling followed by scaling with GL_LINEAR filtering
(3) The nVidia implementation, in GL_SCALED_RESOLVE_FASTEST_EXT mode
(4) The nVidia implementation, in GL_SCALED_RESOLVE_NICEST_EXT mode
(5) Just rendering the image directly to the single-sampled destination
buffer

Observation: Image quality is better in cases 2, 3, 4 and 5 as
compared to case 1. Although extension's implementation meets the
specification's requirements, using it leads to  blocky artifacts
due to nearest filtering.