* Optimize clip logical, change to use GL_GRATER and keep incrace depth
value, so no need to do depth clear after every clip draw.
* Correct geometry bounding box calculation, and make sure the bounds is
larger than all vertices
* Limit drawing area for off-screen RenderPass with correct scissor box
* Fix error when handle GradientTransform calculation. And move the inv
calculation into gradient vertex shader.
* Fix cubic tessellation not close the last point
Since blit msaa framebuffer to another msaa framebuffer may generate
GLError in some platforms. Use normal texture rendering to blit the final
color buffer onto target framebuffer.
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
* 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
