Instead of rendering the swizzled result to the destination texture,
render to another intermediate texture and then CopyTexImage it into the
destination texture.  Rendering to arbitrary mip levels of the destination
texture is not always supported but CopyTexImage-ing is.

BUG=angleproject:1113

Change-Id: I352a1fcc769ce92268744d3e7a6265dd2bfddb37
Reviewed-on: https://chromium-review.googlesource.com/310570Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>