vc4: Handle partial loads/stores of tiled textures.

Previously, we would load out the tile-aligned area, update the raster
copy, and store it back.  This was a huge cost for XPutImage calls to the
screen under glamor.

Instead, implement a general load/store path that walks over the source
x/y writing into the corresponding pixel of the destination (using clever
math from
https://fgiesen.wordpress.com/2011/01/17/texture-tiling-and-swizzling/).
If things are aligned, we go through the previous utile-at-a-time loop.

Improves x11perf -putimage10 performance by 139.777% +/- 2.83464% (n=5)
Improves x11perf -putimage100 performance by 383.908% +/- 22.6297% (n=11)
Improves x11perf -getimage10 performance by 2.75731% +/- 0.585054% (n=145)

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