Some compressed block sizes don't align properly to surface
dimensions. For instance, ASTC_6x6_UNORM populating a 64x64 surface.
Our FrameCapture code was assuming block alignment and was losing
data.

To handle this, round the dimensions up to natural block alignment
before doing any scaling math. This also fixes the problem we've had
of losing small mip levels due to scaling.

Test: Aztec Ruins MEC
Bug: b/160808198
Bug: angleproject:5552
Change-Id: I194cdac87f7361f85539e78f85069b336ffb1f36
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2634205Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Cody Northrop <cnorthrop@google.com>