Improve window decorations in OpenGL scene

When creating the texture containing the window decorations, the drawing code now directly handles the rotation for the left and right decoration, instead of rotating the image after it has been drawn.

The padding, to prevent texture bleeding, is now a fixed value instead of being scaled. With this change, there are no longer visual artifacts for window decorations with rounded corners, when the scaling value is fractional.

Here is a before and after screenshot with scaling set to 150%: