Fix rounded rectangle of GraphicsContext is a little off at the bottom side (fix #3795)

[Gasparoken]

This correction improves the symmetrical appearance of the corners.

However, some issues remains even with this fix:

• The size of input rectangle doesn't match with the output rounded rectangle bounds
• In some cases the rounded rectangle has non symmetrical corners.
• In some cases, some pixels are missing in the stroke of the corners.

Fix https://github.com/aseprite/aseprite/issues/3795

[dacap]

Merged in f89c72e5bfdc6d8923fe0b3fbef15ec38333630a