vulkan: share one zeroed block between empty sparse blocks on AMD

AMD's drivers can't sample images with unbound memory, so large sparse
images are backed whole there, Mario Kart World's nine sparse masks
included (about 2.9 GB, re-uploaded whole for every tile the game maps).

Where the driver supports sparseResidencyAliased, SparseTiles images are
now created with VK_IMAGE_CREATE_SPARSE_ALIASED_BIT and every block is
bound to one zeroed 64 KiB block when the image is created (the image is
cleared once); a block gets memory of its own when data arrives, as on
other drivers. Nothing is ever left unbound. If the GPU renders into such
an image, every block gets memory of its own first, cleared, and the data
is loaded again, so the shared block stays zero.
EDEN_NX2_SPARSE_ALIASED=1 uses the shared block wherever it is supported,
=0 never.

Radeon 780M (proprietary driver 32.0.31041), Mario Kart World title
screen, 200 s: no hang or device loss, world lit, shared memory 12.7 GB
at the end growing 1.8 GB/min, against 18.7 GB and 5.1 GB/min with the
masks backed whole. With every sparse image unbound instead (no AMD
fallback at all) the static world renders dark there, so the other large
sparse images stay backed whole on AMD.

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