Fix GLES sampler precision issue for advanced pipeline

[Icenowy]

Currently we do not add precision description to sampler2D's, and the default precision defined by ESSL spec is lowp.

Experimentation on an Imagination AXM-8-256 GPU shows that the advanced pipeline needs highp sampler2D, otherwise some distant objects will glitch (noticable at the start point of black_forest map).

Manually add precision modifiers for sampler2D to fix the glitch.

Tested on Eswin EIC7700 with X11+EGL+GLES with Imagination DDK 23.2 driver.

P.S. setting mediump instead of highp on sampler2D's seems to be not enough for resolving the glitch; maybe hardwares w/o highp support need to be forbidden from advanced pipeline or some shaders need to be rewritten to be mediump-safe.

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[Icenowy]

Re the P.S.: Looks like highp is core (instead of optional) to ESSL3.0, and as the whole advanced shader-based pipeline requires ESSL3.0, maybe the fallback mediump codepath isn't needed?

[Alayan-stk-2]

Re the P.S.: Looks like highp is core (instead of optional) to ESSL3.0, and as the whole advanced shader-based pipeline requires ESSL3.0, maybe the fallback mediump codepath isn't needed?

@deveee Do you know anything about that?

[deveee]

For GLES2 it uses only very simple irrlicht renderer (can be tested with force legacy device in config.xml). For shader-based pipeline it needs GLES3 for both enabled and disabled advanced lighting. And indeed it seems that highp is core feature and I probably added glGetShaderPrecisionFormat() just to be safe.

Btw there is also

    if (CVS->isGLSL())
        code << "#version 300 es\n";

and isGLSL() should be always true.

Maybe better change after release.