Fix Sprite3D and flipped RenderTarget
Created by: ShyRed
Make Sprite3D fallback on original texture size getting when texture_get-calls return 0.
Flip scissor rectangle when render target is flipped. Without this change, canvas items that rely on clipping won't render correctly / won't render at all.
Fixes #17635 (closed)
Change can be tested with project in #17635 (closed)