This commit adds a new rendering backend, GLES2, and adds a
project setting to enable it.
Currently this backend can only be used on the X11 platform,
but integrating into other platforms is planned.
Added support for 2D shadow casters.
*DANGER* Shaders in CanvasItem CHANGED, if you are using shader in a
CanvasItem and pull this, you will lose them. Shaders now work through a
2D material system similar to 3D. If you don't want to lose the 2D
shader code, save the shader as a .shd, then create a material in
CanvasItem and re-assign the shader.
karroffel
Rémi Verschelde
Juan Linietsky