* restrict the scissor box of composite task
* do not tessellate stroke or fill geometry if there is no Fill or
Stroke color
* use actually transformed curve to calculate polyline count when doing
curve flatten
* Fix error when handle GradientTransform calculation. And move the inv
calculation into gradient vertex shader.
* Fix cubic tessellation not close the last point
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
* 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
