Add partial support for DrawElementsIndirect.
This supports all primitives types except lineloop.
Includes a compute shader for converting 8bit index
buffers to 16bit index buffers where the index buffer range
is defined in a GPU buffer.

Test:
    dEQP.GLES31/functional_draw_indirect_*

Bug: angleproject:3564
Change-Id: Ibe9c55323e46a398f0b703cd8597a72ba6790570
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1792948
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>