Revise WEBGL_shader_pixel_local_storage extension.

[kenrussell]

Add noncoherent usage flag, and renumber enums, to track the current ANGLE extension.

Tested against the revised extension implementation in Chromium.

Associated with https://issues.angleproject.org/issues/40096838 .

[lexaknyazev]

Please make the following changes.

• In the XML:
  • update the getFramebufferPixelLocalStorageParameterWEBGL description to include the new PIXEL_LOCAL_USAGE_WEBGL parameter name.
• In the test:
  • update all calls to framebufferTexturePixelLocalStorageWEBGL to pass zero for usage;
  • update checkInitialValues to check that initial usage value is zero.

[kenrussell]

• In the XML:

  • update the getFramebufferPixelLocalStorageParameterWEBGL description to include the new PIXEL_LOCAL_USAGE_WEBGL parameter name.

Done.

• In the test:

  • update all calls to framebufferTexturePixelLocalStorageWEBGL to pass zero for usage;

Done.

• update checkInitialValues to check that initial usage value is zero.

Done, thanks. This caught a bug in Chromium's extension implementation.