PPM does not store alpha channel, but TIFF does. We add a method
to export RGBA to a TIFF file without compression.
We add the corresponding options to gltf_viewer and
AutomationEngine.
The default export format for both gltf_viewer and AutomationEngine
is now TIFF.
The public dep for mesa has moved on to beyond llvm-18, but the
version we're compiling against is not supported by this change.
We workaround by ensuring that the llvm dep is less than 18 but
consistent with the clang version.
We use Mesa's gallium swrast to render as the driver with
Filament's backend set to GL. We provide a few scripts to parse
the tests (as jsons) and run gltf_viewer to produce the rendering.