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/+/1553967Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Tobin Ehlis <tobine@google.com>