The program textures update needs to call into the incomplete textures
cache. Moving this step into an update mechanism allows us to avoid
some complexity of calling the incomplete textures cache inside the
ProgramVk class.

This enables further refactors for draw call shader patching and line
segment raster.

Bug: angleproject:2598
Change-Id: I0fd81ca58d2641572a2dd444992ccddd99ebc823
Reviewed-on: https://chromium-review.googlesource.com/1127301
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>