- create a new FILAMENT_HAS_FEATURE_TEXTURE_GATHER, which can be used
in the shaders to use textureGather.
- A new TARGET_GLES_ENVIRONMENT and TARGET_GL_ENVIRONMENT in addition
to TARGET_METAL_ and TARGET_VULKAN_, for completeness.
- Rename TARGET_LANGUAGE_SPIRV to FILAMENT_VULKAN_SEMANTICS and add
FILAMENT_OPENGL_SEMANTICS for completeness.
- expose the shader model to shaders as: FILAMENT_SHADER_MODEL_ES30 and
FILAMENT_SHADER_MODEL_GL41
Use these new #define where appropriate.
The main change here is that TARGET_MOBILE is now only used for dealing
with the `mediump` default on mobile.
This change also enables textureGather on Metal.
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