fix shader precision, update Xcode project

data/shaders/texture-mask.glsl

@@ -15,7 +15,7 @@ void main()
 [[fragment]]
 
 uniform sampler2D tex;
-uniform vec2 pattern_offset;
+uniform mediump vec2 pattern_offset;
 
 in mediump vec2 uv;
 out mediump vec4 frag;