[Request]: Add BackgroundBrush support to Dropdown

Description

As it stands Dropdowns do not support brushes, meaning they will be the only large element of the GUI that doesn't have them. Implementation is easy enough, just a matter of doing it. There are probably other ways of going about it, as done in Button simply adding bellow when drawing the background should allow brushes to be used.

if (BackgroundBrush != null)
    BackgroundBrush.Draw(clientRect, backgroundColor);
else
    Render2D.FillRectangle(clientRect, backgroundColor);

Benefits

With background brushes one can easily make the UI look prettier and to their liking. Wouldn't need to rely on the default boxy look.