Create methods for loading shaders depending on GL version

- Load now uses ofShader to load shaders from path,
  so we know for sure it will work the same way.
- Ensure FBOs are set to correct width/height when
  allocate is called.
- Can set shader source for GL ES/2/3 by setting
  specific strings in constructor, or by calling
  loadVersioned.
- Can reload shaders from file without having to recompile.