Use GLES fragment shader as filters

[chenzx]

Use SVG filters is not flexible enough, i pref using custom GLES fragment shader for filter, which provides most flexibles while not need to switch to WebGL/WebGPU.

The background: I'm using canvas2d to build a web-based UI editor framework, which is not a build-from-scratch, but mitigation from an old legacy C++ codebase. For some reason, compiling to WebAssembly + WebGL is too heavy, & i prefer using canvas2d first...