The linear gradients are broken

Created by: xTibor

I tried to use them recently and I found out that the rendering routine is fairly broken:

• The axis-aligned gradients don't respect the starting point parameters.
• The axis-aligned gradients don't render correctly when the ending point precedes the starting point.
• Non axis-aligned gradients render in an incorrect angle.
• Non axis-aligned gradients mirror themselves over the starting point.

Test app: https://gist.github.com/xTibor/ff1b1a040a0716338d73655ff85d1229

It might worth a rewrite at this point. I could tackle it if needed.