wip: mesa: Enable EXT_shadow_samplers for GLES2
I thought this was just the funny GLES spelling of the extn name, but there's also some ESSL bits you need to add. Most of which you could probably yoink from the old Unity glsl-optimizer (which itself yoinked most of the GLSL compiler from Mesa):
https://github.com/aras-p/glsl-optimizer/commit/94a9b2959b885dba4e355aa344adb4d472b00f18
Signed-off-by: Adam Jackson ajax@redhat.com