Load external shader file at compile time via @:glFragmentSourceFile and @:glVertexSourceFile

[giuppe]

This PR adds a way to load an external file to be used as a shader (fragment or vertex).

Usage:

class MyShader extends Shader
{
// ...
@:glFragmentSourceFile("./shaders/myShader.frag")
// ...
}

This could help separate the actual GLSL code from its OpenFL management. Most IDEs can correctly highlight/format the syntax of an external shader file, while long strings (as the @:glFragmentSource() argument) are usually rendered as a uniform color and must be formatted by hand; so this could be a nice quality of life change.

This PR is changing the ShaderMacro.build() function so that:

• it would check for the metas glFragmentSourceFile, :glFragmentSourceFile, glVertexSourceFile, and :glVertexSourceFile;
• if one of the metas is found and the argument file exists, its content will be set as the glFragmentSource/glVertexSource, overriding the @:glFragmentSource/@:glVertexSource argument (if defined).
• if the file does not exist or cannot be read, an error is returned at compile time.

[T1mL3arn]

Completely and totally upvote this. Why is it still not merged?