Vulkan:Return HW's SUBPIXEL_BITS Plumb the physical HW's subPixelPrecisionBits limit to be returned in the query for GL_SUBPIXEL_BITS. Default value remains 4 for all other backends. Bug: angleproject:3351 Change-Id: I5564e5090e7211b8daeaa91ea30eceb23c5ea227 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1553967 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Tobin Ehlis <tobine@google.com>