Add draw_rectangle_ex function and example for using it

[vdrn]

This function merges functionalities of draw_rectangle and draw_rectangle_lines, and adds:

• rotation with pivot
• rounded edges
• horizontal and vertical skewing
• 4 color gradient fill

PS. also fixed error in docs for draw_texture_ex. Not sure what's the best wording, but pivot is in same coordinate system as x and y args.

[musjj]

@not-fl3 Hey can you take a look at this PR? Round-cornered rectangles would be nice to have.

[not-fl3]

Oh, sorry for delay! On it!