hide cursor on canvas, add mouse focus event, brush preview solid when small

src/shader.cpp

@@ -440,6 +440,7 @@ bool check_uniform_uniqueness()
         const_hash("colorize"),
         const_hash("dual_alpha"),
         const_hash("use_fragcoord"),
+        const_hash("draw_outline"),
     };
     std::sort(v.begin(), v.end());
     int last = 0;