gg: optimise the draw_rounded_rect_filled function

[Linklancien]

This PR aims to optimise the draw_rounded_rect_filled function by changing how it works:
Now instead of drawing four corner and 3 rectangle, it draw the top part, the middle part using a rectangle and the bottom part

The top and bottom part are drawn as follow:
For each angle, you place a new point on each side, right and left using the sgl.begin_triangle_strip() primitive

[Linklancien]

To check if this work as well as before, a new function has been added in #26389, that way we can compare if it doesn't break anything

[spytheman]

Thank you @Linklancien 🙇🏻 .

I could only test on macos, the result is that the before/after renders look the same to me 🥳, even though the code is less, and generates a smaller number of vertices.