Add a version directive to the shaders

src/gfx/mask.frag

@@ -1,3 +1,5 @@
+#version 120
+
 uniform sampler2D texture;
 uniform sampler2D mask;
 uniform vec2 offset;

src/gfx/mask.vert

@@ -1,3 +1,4 @@
+#version 120
 
 uniform mat4 trans;