Scaleable GUI elements

[matshou]

Custom GUI elements should be scalable.

[matshou]

@JTK222 provided two implementation samples on MMD:

• Using AbstractGui.blit method:

  blit(double x0, double y0, int destWidth, int destHeight, float u0, float v0, int srcWidth, int srcHeight, int texWidth, int texHeight)

• Using RenderSystem:

  RenderSystem.pushMatrix(); //Push current transformation matrix to the stack
  RenderSystem.translate(0, 50, 0); //Ofset the matrix by 50 on the y axis.
  //Everything rendered here will be offset 50 pixels downwards when in a gui
  RenderSystem.popMatrix(); //Reset the matrix to the state of the last pushMatrix

[matshou]

Reference to mapped blit parameters: https://gist.github.com/gigaherz/f61fe604f38e27afad4d1553bc6cf311