Previously, when a material was expilicitly built for feature level 0, it was
necessary to write ESSL 1.0 code which was incompatible with the OpenGL feature
level 1 implementation in Filament. Rather than adjust the Filament
implementation so that feature level 1 uses the same workarounds as feature
level 0 to emulate GLES 3.0+ features, it's more runtime-efficient to have matc
embed ESSL 1.0 and 3.0 shaders as separate chunks and load the corresponding
one based on the active feature level.
Feature level 0 material shaders must still be written in what is effectively
ESSL 1.0. To assist with this, a small set of compatibility definitions are
introduced when building the ESSL 3.0 variant. These are virtually all
textureXXX() functions, and end up either optimized out or inlined by glslang.
One final effect of this change is that external and 3D samplers are now
properly supported in feature level 0 materials.
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