feat: parameterize stroke color and thickness

shaders/triangle.vert.glsl

@@ -2,10 +2,14 @@
 precision mediump float;
 
 layout(location = 0) in vec2 aPos;
+layout (location = 1) in vec4 aColor;
 
 uniform mat4 uProjection;
 
+out vec4 vColor;
+
 void main()
 {
     gl_Position = uProjection * vec4(aPos, 0.0, 1.0);
-}
+    vColor = aColor;
+}