Switch mobile shader to use default mediump precision.

[DarioSamo]

For now this is a pretty basic PR towards our goal of switching mobile to FP16. I'll be looking at the output in the mobile vendor shader compilers more in detail to see where we can edge out some gains.

Even as it is, without showing much in the way of visual regressions, this PR gives a small performance improvement (2 to 3%) with a very straightforward change.

Will keep this as a draft until I dive deeper into the compiled output and tweak the shader.

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Contributed by W4 Games. 🍀

[Calinou]

Should we have a project setting to force highp in core shaders? We had this in GLES2 in 3.x, which was occasionally used to fix rendering issues in projects running on mobile.

At the same time, it's a band-aid fix that negates the performance improvements from using mediump/lowp in the appropriate locations, so maybe it's better to start from a clean slate.

Also, does this PR affect shaders written by users, either GDShader or GLSL? See also #59990.

[DarioSamo]

Also, does this PR affect shaders written by users, either GDShader or GLSL? See also #59990.

All user variables have the highp prefix to avoid issues in user code (if not specified).