Add GLSL/ESSL validator/translator support for GL_EXT_shader_framebuffer_fetch. BUG=angle:834 Change-Id: I2d4e25909a8e1266b9bb7f1d4421324143157c8a Reviewed-on: https://chromium-review.googlesource.com/231032 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Erik Dahlström <ed@opera.com>