Adds linear gradient support to 2D meshes in the canvas widget.

glow/src/shader/common/triangle.vert

@@ -1,11 +1,9 @@
 uniform mat4 u_Transform;
 
 in vec2 i_Position;
-in vec4 i_Color;
-
-out vec4 v_Color;
+out vec2 raw_position;
 
 void main() {
     gl_Position = u_Transform * vec4(i_Position, 0.0, 1.0);
-    v_Color = i_Color;
+    raw_position = i_Position;
 }