[Impeller] Use the ES profile for shaderc ingestion

[bdero]

This is a step towards being able to control floating point precision. The GLSL 4.6 spec ignores precision (and so does glslang in this mode). The compiler is still not emitting half precision on Metal, so something else probably needs to be adjusted (perhaps the shaderc_env or spirv_target) since it appears to have support for doing so internally.

All of the shaders needed to be modified because explicitly specifying precision (and explicitly specifying conversions between different precisions) is mandatory for GLSL ES 3.2. If we end up needing to reverse this change for some reason, none of the shaders will need to be changed to do so, as all of the changes are also compatible with GLSL 4.6

[flutter-dashboard[bot]]

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[bdero]

Might be worth experimenting with this a bit more before landing -- we may or may not end up needing it.

[chinmaygarde]

@bdero Do you still intend on landing this. If not, let's move this to the icebox?

[bdero]

@chinmaygarde I don't think we should land this right now -- moved to ice box. I didn't find a way to tease the compiler into emitting precision info and this needs a deeper look at shaderc internals.