Godot crt-lottes shader

This is a port of the libretro crt-lottes GLSL shader to Godot shading language.

The original source is: https://github.com/libretro/glsl-shaders/blob/master/crt/shaders/crt-lottes.glsl

There is a basic example scene, consisting in just a ViewportContainer, with a Viewport containing a "game" scene. The ViewportContainer has the shader asigned to its material property, and must be upscaled using the rect_scale property, preferably using an integer factor (x2, x3, x4...).

In the shader params must specify:

• input size: resolution of source image. In this example, 256x224.
• output size: resolution of final image. In this example it is scaled x4, so it is 1024x896.
• texture size: must be equal to input size.

In project settings, disable any window stretching mode.

The original Lottes shader was made by Timothy Lottes and is public domain, and this port is also.