1. Viewport::get_visible_rect().position is always zero.
So Control::get_window_rect is identical to Control::get_global_rect.
Remove Control::get_window_rect since it is not used in the source code.
2. sqrt(a * a) = abs(a) for doubles
3. Simplify affine_inverse combination
4. Simplify calculation in shaders
Add "generate_mipmap" font import option.
Add some missing features to the Sprite3D.
Move BiDi override code from Control to TextServer.
Add functions to access TextServer font cache textures.
Add MSDF related flags and shader to the standard material.
Change standard material cache to use HashMap instead of Vector.
-Mesh2D now works
-MultiMesh2D now works
-Polygon2D now works
-Added hooks for processing 2D particles
-Skeleton2D now works
2D particles still not working, but stuff needed for it is now implemented.
-Used a more consistent set of keywords for the shader
-Remove all harcoded entry points
-Re-wrote the GLSL shader parser, new system is more flexible. Allows any entry point organization.
-Entry point for sky shaders is now sky().
-Entry point for particle shaders is now process().
