Big slowdown on using `rounded_rectangle` with text boxes

[joereynolds]

Describe the bug
I've got some dialogue boxes in my game. when I call set_text() on a textbox with a rounded_rectangle shape, it lags very noticeably.

To Reproduce
Steps to reproduce the behaviour:

1. Create a textbox with a rounded rectangle
2. Call set text
3. The setting of text is laggy

Expected behaviour
It loads instantly like it does with other shapes

Screenshots

And further into the library itself: