EXT_multisampled_render_to_texture was causing perf
degradation through multiple unnecessary creation/destruction
of the shadow multisampled texture. This is solved by keeping around
the multisampled texture, so both will be available.

Bug: chromium:1015031
Change-Id: I44deee67c02c19bc8749d3ed0d45661f52902e72
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1872618Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>