[Yulong Wang (fs-eire)]

Description: This PR contains several changes to the WebGL backend framework code. The major changes are:

• unify concepts ProgramInfo and RunData for operator implementations. Previously it's designed based on the assumption that each operator will be implemented by using exactly 1 WebGL shader program, which is proved to be false. The old operator interface is replaced by a function-based one, using WebGLInferenceHandler.run() to run a program, which can be called multiple times in one operator's implementation.
• remove createProgramInfo and createRunData and use one implementation function.
• operator resolves to the implementation function ( instead of the instance based Operator interface)
• Hide TextureLayout and TextureData from operator implementation. Now they are WebGL backend internal data structures.
• introduce ProgramInfoLoader for caching reusable program info. (WIP)

The behavior should not be changed before vs. after the refactoring.

Benefit of this change:

• operators are much easier to develop (ie. implement new operator, or maintain existing ones), because of the simplification on interfaces and concepts.
• artifacts cache will work as expected for all programs. (before this refactoring, the cache breaks when multiple shader programs are used in one operator implementation. )
• cleaned up blockers for some future features (eg. texture based const-folding)

[Yulong Wang (fs-eire)]

wrong source branch. closed and replaced by #8586