test: Make stress-test more likely to actually composite something
stress-test current almost never composites anything because the clip
rectangles and transformations are such that either
_pixman_compute_composite_region32() or analyze_extent() will return
FALSE.
Fix this by:
- making log_rand() return smaller numbers so that the clip rectangles
are more likely to be within the destination image
- adding rand_x() and rand_y() functions that pick positions within an
image and using them for positioning alpha maps and source/mask
positions.
- making it less likely that clip regions are used in general
These changes make the test take longer, so speed it up a little by
making most images smaller and by reducing the maximum convolution
filter from 17x19 to 3x4.
With these changes, stress-test reveals a crash in iteration 0xd39
where fast_composite_tiled_repeat() creates an indexed image without a
palette.