RenderTargetVk::getImageExtents() was always returning the base level
extents of the image it was viewing.  This resulted in incorrect extents
being provided to various framebuffer- and renderpass-related
declarations.  The function is renamed to `getExtents()` and returns the
appropriate mip extents.

Bug: angleproject:2361
Change-Id: I059a8d19fcb140c9095107d935aa3e5cb1852fc2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1537327
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>