Add support for WebGL2 read and draw buffer settings
Adds support for the `ReadBuffer` and `DrawBuffers` WebGL2 calls and the related parameter getters. See: - https://www.khronos.org/registry/webgl/specs/latest/2.0/#3.7.2 - https://www.khronos.org/registry/webgl/specs/latest/2.0/#3.7.4 - https://www.khronos.org/registry/webgl/specs/latest/2.0/#3.7.11
