Support partial update in WebGL

[Kiikurage]

Currently, WebGL backend does NOT support partial update of output matrix, because WebGL backend render rectangle polygon to whole of output buffer. However, it's possible to support this feature by adjusting polygon's position appropriately.

Partial update is mainly used in concatenation operation, and concatenation is used for texture-splitting. Therefore, this feature may greatly improve the performance in WebGL backend for various situations.

[lygstate]

is partial Update are possible in webgl？

[lygstate]

@Kiikurage What's the hard part to doing partial update?