[Perf] (ShaderTranspiler, ProgramState): memoize the glslang parse verdict so a repeated compile skips the parse

MobileGL/Config.h

@@ -203,13 +203,14 @@ namespace MobileGL::MG_Config {
         // immediately stay serial by their own construction). Off by default; never
         // advertise it.
         QuirkOverride AsyncOptimisticShaderStatus = QuirkOverride::Auto;
-        // MOBILEGL_SHADER_CACHE: the two-level, in-memory shader translation memo
-        // (MG_Util/ShaderTranspiler/TranslationCache.h) - L1 memoizes a linked
-        // program's sanitized SPIR-V, L2 memoizes DirectGLES's emitted ESSL. Auto
-        // is ON; ForceOff turns BOTH levels off and makes every translation run
-        // from scratch. The escape hatch exists because a wrong cache hit is a
-        // silently miscompiled shader: if a device ever renders differently with
-        // the cache on, one run with this falsy says so.
+        // MOBILEGL_SHADER_CACHE: the three-level, in-memory shader translation memo
+        // (MG_Util/ShaderTranspiler/TranslationCache.h). The levels follow the GL
+        // entry points - L1c memoizes one glCompileShader's PARSE VERDICT, L1 a
+        // linked program's whole front end, L2 DirectGLES's emitted ESSL. Auto is
+        // ON; ForceOff turns ALL THREE off and makes every translation run from
+        // scratch. The escape hatch exists because a wrong cache hit is a silently
+        // miscompiled shader: if a device ever renders differently with the cache
+        // on, one run with this falsy says so.
         QuirkOverride ShaderTranslationCache = QuirkOverride::Auto;
     };
     extern FeaturesTable Features;