Fixed several other shader validation errors caught when running
vello_shaders natively on Metal.
These were primarily caused by reading an invalid drawtag while
accessing the scene buffer. Scene buffer access in the offending
pipelines now initialize the draw tag to DRAWTAG_NOP if an
invocation ID would land beyond the valid index range of encoded draw
objects.
* replace one_minus_focal_x and abs_one_minus_focal_x variables with the actual expressions
* replace division by r^2-1 with multiplication by reciprocal
* revert chain selects to branchy code for clarity. Branching is dynamically uniform so shouldn't affect performance
* add suggested comment describing gradient kind/flags constants
Adds full support for COLRv1 radial gradients based on the two-point conical gradient algorithm at https://skia.org/docs/dev/design/conical/
Also adds robustness to degenerate cases in gradient encoding:
* Radial where p0 == p1 && r0 == r1 renders transparent solid
* Empty stops render as transparent solid
* Single stop renders as solid
This patch implements the pad, repeat and reflect extend modes for gradient brushes. Adds a new example demonstrating the functionality.
Also fixes a few bugs:
* Clamps alpha in blend.wgsl for the `blend_compose` function. The `Plus` mode was generating `alpha > 1.0` leading to incorrect rendering.
* Small change to radial gradients in fine.wgsl to reject pixels outside the cone when the circles don't nest. This requires further work to properly extend the cone when one of the radii is not 0.
* Add counts to offsets when comparing against buffer size limits
* Remove multiplication by 4 in blend buffer allocation (we use units of u32)
* Move buffer sizes from BumpAllocators to Config
* Add comments about early exit
* Move the vello crate to the root of the crate
* Add warning that README is work in progress
* Add newline in warning
* Move the unlicense into the shader folder
* Fixup wgsl-analyzer include paths
