Add builtin shaders

[lukexor]

Pixelate (or nearest neighbor) and Ntsc (by bisqwit) are already as AV filters provided but no shaders are used for those yet. See #254

AV filters can also be applied independently from post processing shaders.

An initial list to get started:

• crt (eg. scanlines)
• anti-alias
• bicubic
• bilinear
• pal
• inverted
• black-and-white

Adding an shader requires:

• adding the shader code to the shaders folder
• Adding an enum variant to Shader enum
• Adding a switch statement for creating the shader resources and rendering the frame
• Adding the new variant to the shader_radio method for the gui