Strength-reduce BILINEAR filter to NEAREST filter for identity transforms

Strength-reduce BILINEAR filter to NEAREST filter for identity transforms

An image with a bilinear filter and an identity transform is
equivalent to one with a nearest filter, so there is no reason the
standard fast paths shouldn't be usable.

But because a BILINEAR filter samples a 2x2 pixel block in the source
image, FAST_PATH_SAMPLES_COVER_CLIP can't be set in the case where the
source area is the entire image, because some compositing operations
might then read pixels outside the image.

This patch fixes the problem by splitting the
FAST_PATH_SAMPLES_COVER_CLIP flag into two separate flags
FAST_PATH_SAMPLES_COVER_CLIP_NEAREST and
FAST_PATH_SAMPLES_COVER_CLIP_BILINEAR that indicate that the clip
covers the samples taking into account NEAREST/BILINEAR filters
respectively.

All the existing compositing operations that require
FAST_PATH_SAMPLES_COVER_CLIP then have their flags modified to pick
either COVER_CLIP_NEAREST or COVER_CLIP_BILINEAR depending on which
filter they depend on.

In compute_image_info() both COVER_CILP_NEAREST and
COVER_CLIP_BILINEAR can be set depending on how much room there is
around the clip rectangle.

Finally, images with an identity transform and a bilinear filter get
FAST_PATH_NEAREST_FILTER set as well as FAST_PATH_BILINEAR_FILTER.

Performance measurementas with render_bench against Xephyr:

Before

*** ROUND 1 ***
---------------------------------------------------------------
Test: Test Xrender doing non-scaled Over blends
Time: 5.720 sec.
---------------------------------------------------------------
Test: Test Xrender (offscreen) doing non-scaled Over blends
Time: 5.149 sec.
---------------------------------------------------------------
Test: Test Imlib2 doing non-scaled Over blends
Time: 6.237 sec.

After:

*** ROUND 1 ***
---------------------------------------------------------------
Test: Test Xrender doing non-scaled Over blends
Time: 4.947 sec.
---------------------------------------------------------------
Test: Test Xrender (offscreen) doing non-scaled Over blends
Time: 4.487 sec.
---------------------------------------------------------------
Test: Test Imlib2 doing non-scaled Over blends
Time: 6.235 sec.