Add a custom shader for correct pixel-art upscaling

[drakmaniso]

The best filtering for upscaling pixel-art is neither "nearest" nor "linear", but a combination of the two (i.e. most pixels use "nearest", but screen pixels situated at the border between virtual pixels use linear).
References:

• Manual texture filtering for pixelated games in WebGL
• Scaling Pixel Art Without Destroying It
• ShaderToy: smooth pixel art zoom & scroll

Unfortunately, in the current state of Bevy (0.8), it doesn't seem possible to customize the shader used by the sprite system (see issue bevyengine/bevy#1738).

An alternative would be to implement a custom sprite system, as done in https://github.com/ManevilleF/bevy_sprite_material, but it's probably best to wait for Bevy to provide a more elegant solution.

[cloud303-cholden]

@drakmaniso Just curious, is this why the cloud pixels in flappin look thinner when the background is scrolling? I've been trying to find a solution to this for my own game.