[rlgl] fix vertex calculation precision in vertex shader for depth buffer

[arlez80]

Insufficient precision in vertex calculations causes depth calculation issues when 3D rendering, resulting in Z-fighting or clipping error.
I fixed specify highp to ensure sufficient precision.

The screen flickers with every camera movement, I'm took two screenshots of the issues occurring.
environment: RG40XXH (Allwinner h700; Quad-core ARM Cortex-A53; G31 MP2 GPU)

the ground is clipped.

the character's some polygon is z-fighting. the ground depth is weird also.

[maiconpintoabreu]

Hi @arlez80 the default is already highp no?
Reference: https://www.khronos.org/files/opengles20-reference-card.pdf

[arlez80]

@maiconpintoabreu yes the glsl default is highp.
but the default precision set mediump at here or here.
therefore, I have set highp for the vertex variable and model-view-projection matrix.

[raysan5]

@arlez80 Afair, not all platforms support highp, specially for OpenGL ES 2.0, also specifying the precision at beginning should be enough. I prefer to avoid this change.

[arlez80]

@raysan5
I have a proposal, how about separate them by macro?

like:

// ...
#if defined(GRAPHICS_API_USE_HIGHP_VERTEX)
"in highp vec3 vertexPosition;      \n"
#else
"in vec3 vertexPosition;            \n"
#endif
// ...