Add SkImageInfoValidConversion() and SkImageInfoIsValid

The idea is share these standards for the following:
SkImage::readPixels()
SkCanvas::readPixels()
SkCanvas::writePixels()
SkBitmap::readPixels()
SkPixmap::readPixels()

On the raster side, SkPixmap::readPixels() is the right
place to check, because all raster calls go through
there eventually.  Then at lower levels (ex: SkPixelInfo),
we can assert.

There's not really a unifying location for gpu calls,
so I've added this in multiple places.  I haven't really
dug into the gpu code to SkASSERT() on invalid cases
that we will have already caught.

Follow-up work:
Similar refactor for SkReadPixelRec::trim().
Code cleanup in SkPixelInfo::CopyPixels()

BUG=skia:6021

Change-Id: I91ecce10e46c1a6530f0af24a9eb8226dbecaaa2
Reviewed-on: https://skia-review.googlesource.com/6887
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Mike Reed <reed@google.com>
9 files changed