Implement GLSL additions for ANGLE_shader_pixel_local_storage Specs out, implements, and thoroughly tests the GLSL additions for ANGLE_shader_pixel_local_storage. Adds a simple transformation that rewrites PLS directly into shader images. Updates the existing PLS tests to use the newly built-in PLS features and ensures they continue passing. For now, applications call glBindImageTexture to configure their pixel local storage. The OpenGL ES API side of this extension will follow shortly. Bug: angleproject:7279 Change-Id: I141183069b5cbfcca01cbb77b5b36d3e5f834bf5 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3761876 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Chris Dalton <chris@rive.app> Reviewed-by: Kenneth Russell <kbr@chromium.org>