improve brush preview

src/shader.h

@@ -37,6 +37,10 @@ enum class kShaderUniform : uint16_t
     LightDir = const_hash("light_dir"),
     Mode = const_hash("mode"),
     Ambient = const_hash("ambient"),
+    PatternScale = const_hash("pattern_scale"),
+    PatternBright = const_hash("pattern_bright"),
+    PatternContrast = const_hash("pattern_contr"),
+    Colorize = const_hash("colorize"),
 };
 
 enum class kShader : uint16_t