Make SkSL GLSL generator declare sk_FragColor inout when EXT fb fetch is used.

Cherry-pick of dc09213b17cfd100e1c1859387b47e3045a36010 to skqp/dev

No-Tree-Checks: true
No-Try: true
No-Presubmit: true
Change-Id: If89b4fbb917cb0832411491811fd9a024f5a3067
Reviewed-on: https://skia-review.googlesource.com/118587
Reviewed-by: Derek Sollenberger <djsollen@google.com>
9 files changed