BeginScissorMode checks for render texture to avoid using GetWindowScaleDPI

[gulrak]

When on a window with active FLAG_WINDOW_HIGHDPI window flag or on macOS the implementation of BeginScissorMode uses GetWindowScaleDPI to get scaling and uses it on the scissor rect.

If using BeginScissorMode inside of an open BeginTextureMode/EndTextureMode this scaling is
wrong as a RenderTexture is created with a defined resolution and doesn't share any implicit window dpi scalings.

I fixed this by checking if currently a frame buffer is bound to detect the texture mode. If there is a binding the else path of the original implementation will be used to avoid the wrong scaling.

[raysan5]

@gulrak Thanks for the review! Just added CORE.Window.usingFbo flag to register when an FBO is being used, we can check against it. I'm merging and reviewing it myself!

[gulrak]

Sorry, I was so happy for it to be merged that I missed you flipped the logic when changing the way the texture mode is detected, now it always uses GetWindowScaleDPI if we are in texture mode, but my solution (and the one needed) was the opposite (So it needs to be !CORE.Window.usingFbo.