Optimize UI rendering

[mrDIMAS]

UI rendering is currently very unoptimal - pretty much every drawing command generated by the UI is executed "as-is" by the renderer, which causing insane amount of draw calls. For example the editor itself can easily generate 2k of draw calls.

Optimization should merge drawing commands (when possible) into a bigger one, essentially it should merge vertex/index buffers together into bigger chunks. However, it is only possible when drawing commands does not use unique textures and when bounds of drawing command is less or equal to clipping bounds (otherwise the renderer must use scissor test, which breaks rendering batch).