Add ifStatic shader keyword

The stage-level shader option ifStatic <expression> evaluates an
expression at load time and disables the stage if the expression
evaluates to a falsey float value.

Author: slipher

src/engine/renderer/tr_shader.cpp

@@ -2105,6 +2105,7 @@ static bool ParseStage( shaderStage_t *stage, const char **text )
 	char         buffer[ 1024 ] = "";
 	bool     loadMap = false;
 	bool loadAnimMap = false;
+	bool staticEnabled = true;
 
 	stage->convertColorFromSRGB = convertColorFromSRGB_NOP;
 
@@ -2130,6 +2131,12 @@ static bool ParseStage( shaderStage_t *stage, const char **text )
 		{
 			ParseExpression( text, &stage->ifExp );
 		}
+		else if ( !Q_stricmp( token, "ifStatic" ) )
+		{
+			expression_t expr;
+			ParseExpression( text, &expr );
+			staticEnabled = !!RB_EvalExpression( &expr, 1.0f );
+		}
 		// map <name>
 		else if ( !Q_stricmp( token, "map" ) )
 		{
@@ -3339,6 +3346,12 @@ static bool ParseStage( shaderStage_t *stage, const char **text )
 		}
 	}
 
+	if ( !staticEnabled )
+	{
+		// parsing succeeded, but stage disabled
+		return true;
+	}
+
 	// parsing succeeded
 	stage->active = true;