Enforce our rules about valid images when making textures

I'm working to make GrUploadPixmapToTexture more robust
and easier to follow. This is one step on that journey.

BUG=skia:

Change-Id: I3ac39057e20ff8249843bb41ceca25f629aff31c
Reviewed-on: https://skia-review.googlesource.com/7037
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Matt Sarett <msarett@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
1 file changed