Since we choose MSAA, no need to calculate edge alpha during fragment
stage. So this commit removed the alpha attribute and related code:
* Remove the alpha attribute in vertex data.
* Change position type from `vec3` to `vec2` in all shader code.
* Remove alhpa multiplication in all fragment shaders
Since we choose multisample antialiasing, use stencil then cover to
render polygon can get better performance.
Also the code is much easier to understand.
* add new render task to do stencil and cover rendering which is a
fallback rendering method to handle cases that trianglation tessellation
failed
* add a new tessellator to generate stencil and cover vertex mesh
* merge vertices that are close enough before tessellation
* append return branch in tessellation to prevent dead loop caused by floating point precision
* Use uniform block to pack all color informations
* Move the actual gl draw call into GlRenderer::sync function, so all
data is been uploaded into GPU
* Make GlRenderTask simple and generic for direct gl draw
