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.
* 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
