mesa: implement glGetShaderPrecisionFormat()

Drivers should override the default range/precision info as needed.
No drivers do this yet.

src/mesa/main/shaderapi.c

@@ -1628,12 +1628,51 @@ void GLAPIENTRY
 _mesa_GetShaderPrecisionFormat(GLenum shadertype, GLenum precisiontype,
                                GLint* range, GLint* precision)
 {
+   const struct gl_program_constants *limits;
+   const struct gl_precision *p;
    GET_CURRENT_CONTEXT(ctx);
-   (void) shadertype;
-   (void) precisiontype;
-   (void) range;
-   (void) precision;
-   _mesa_error(ctx, GL_INVALID_OPERATION, __FUNCTION__);
+
+   switch (shadertype) {
+   case GL_VERTEX_SHADER:
+      limits = &ctx->Const.VertexProgram;
+      break;
+   case GL_FRAGMENT_SHADER:
+      limits = &ctx->Const.FragmentProgram;
+      break;
+   default:
+      _mesa_error(ctx, GL_INVALID_ENUM,
+                  "glGetShaderPrecisionFormat(shadertype)");
+      return;
+   }
+
+   switch (precisiontype) {
+   case GL_LOW_FLOAT:
+      p = &limits->LowFloat;
+      break;
+   case GL_MEDIUM_FLOAT:
+      p = &limits->MediumFloat;
+      break;
+   case GL_HIGH_FLOAT:
+      p = &limits->HighFloat;
+      break;
+   case GL_LOW_INT:
+      p = &limits->LowInt;
+      break;
+   case GL_MEDIUM_INT:
+      p = &limits->MediumInt;
+      break;
+   case GL_HIGH_INT:
+      p = &limits->HighInt;
+      break;
+   default:
+      _mesa_error(ctx, GL_INVALID_ENUM,
+                  "glGetShaderPrecisionFormat(precisiontype)");
+      return;
+   }
+
+   range[0] = p->RangeMin;
+   range[1] = p->RangeMax;
+   precision[0] = p->Precision;
 }