GLSL Loader for webgl-rc

Shader file

```glsl /* include absolute path 'project_dir/glsl/lib/color.glsl' */

include

/* include relative path './common/bezier.glsl' */

include "./common/bezier.glsl"

void main() { ... } ```

Rust file

```rust use webglrc::loadglsl;

const fragmentsource: &str = loadglsl!("fragment.glsl");

```