[Feature] Implement FlixelShader and FlixelUpdatedShader

[stringdotjar]

Implement FlixelShader and FlixelUpdatedShader classes for converting Flixel .frag files into something that libGDX can understand.

Requirements:

• Use the FlixelReflectUtil to allow changing uniforms at runtime (for FlixelUpdatedShader).
• Inject a multi-line String at the top of the shader when processing it, including many #defines to rename the Flixel GLSL variables/methods/constants to ensure Flixel .frag shaders work out of the box and libGDX can use it.
• Allow a toggle to also use regular libGDX shaders and .vert shaders to allow compatibility with existing libGDX games.