anv: Add support for VK_EXT_sample_locations
Replacing !1887 (merged)
I took into account @Danil's review and also rebased/reworked things a bit and finally squashed some commits so that it's not too many commits.
CI all green : https://mesa-ci.01.org/djdeath_vulkan/builds/499/group/63a9f0ea7bb98050796b649e85481845