Performance issues with background-image property

[KurtGokhan]

Currently the background-image property has some performance issues that drop fps significantly when used abundantly.

There are two main reasons:

• NotifyStencilStateChanged is called needlessly. Discord link
• Background image draw calls are not batched because of non-sharing materials. Discord link

Additionally, performance can be improved for box-shadow with this technique

[KurtGokhan]

Looks like all of these issues will be solved once I implement mesh-based background/border/shadow instead of shader-based.

[KurtGokhan]

This is being worked on the branch https://github.com/ReactUnity/core/tree/mesh-based-bg-shapes

[KurtGokhan]

Implemented all border, border-image, background and similar features using meshes now, instead of shaders. This will improve performance a lot, as well as allow anti-aliasing.