Use high precision in generated ESSL (#2792)

## Change

- Specify `highp` to use 32-bit precision with ESSL

Fixes: #2655, #2654.

### Details

High Precision Change

The minimum ESSL version required is 3.0 , where `highp` precision is mandatory in fragment shaders, so the switch is safe.
Instead of IEEE 754 half‑precision (16‑bit) we now use single‑precision (32‑bit). This avoids cases of underflow/overflow and reduces accumulated math errors (for Std surface, OpenPBR and glTF).

Author: kwokcb

source/MaterialXGenGlsl/EsslShaderGenerator.cpp

@@ -27,7 +27,9 @@ void EsslShaderGenerator::emitDirectives(GenContext&, ShaderStage& stage) const
 {
     emitLine("#version " + getVersion(), stage, false);
     emitLineBreak(stage);
-    emitLine("precision mediump float", stage);
+    // ESSL 3.0+ is used where highp float is considered mandatory.
+    // (See https://registry.khronos.org/OpenGL/specs/es/3.0/GLSL_ES_Specification_3.00.pdf)
+    emitLine("precision highp float", stage);
     emitLineBreak(stage);
 }