Move internal corner rendering to separate shader

[WillPower3309]

We can get some minor perf improvements by clipping the entire window box from our decoration node rendering, and then render the four corners of the windows with the decoration blend shaders

[WillPower3309]

We could move external corner rounding to a separate shader as well! This is coupled with #72

[ErikReider]

Can we close this as well?

[WillPower3309]

Not yet, I want to play with this first :) I have some optimization ideas

[WillPower3309]

Not yet, I want to play with this first :) I have some optimization ideas

Sort of a two step approach, where we render the internal corners in a separate step so we can take that perf hit of accounting for those corners in a smaller region