Consider EXT_shader_pixel_local_storage

In the WebGL WG conference call of 2022-02-10, the idea of exposing [EXT_shader_pixel_local_storage](https://www.khronos.org/registry/OpenGL/extensions/EXT/EXT_shader_pixel_local_storage.txt) was discussed.

The point was made that this is a least-common-denominator programmable blending extension, and can actually be implemented on top of EXT_shader_framebuffer_fetch, which is being rejected in #3378 .

Start by reading the linked EXT_shader_pixel_local_storage specification and the discussion referenced in #3378. The issue does not name files, tests, or an implementation entry point; work is complete only after the project decides whether and how this extension should be exposed.

Written by the indexing model from the issue text.