Remove GrGLGpu/GrGLCaps readPixelsSupported.

This code checks the external format/type that will be used with
glReadPixels. It tests for the values inherently allowed by the GL
and for implementation values.

It would need to be refactored to use formats and color types rather
than GrPixelConfig.

However, the code that calls GrGpu::readPixels and
GrGpu::transferPixelsFrom already tests
GrCaps::supportedReadPixelsColorType. The GrGLCaps override only allows
format/type combinations allowed by ES without implementation queries,
which is less permissive than the deleted code.

We may get more permissive again but will probably follow a different
pattern and do this after the format/colortype tables are built out in
GrGLCaps.

Bug: skia:6718
Change-Id: I36d9ab496dfe71045dcbce84200a9de3e93440bf
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/226840
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
4 files changed