The current flag did not work because float shader flags are not
supported. It was also not very useful because it was per-shader
and did not use the alpha cutoff property of glTF materials.
Instead, let's turn the shader flag into an enable/disable boolean,
and add a scalar material property named "alphacutoff" that gets
read by the glTF importer.
When the alphaCutoff flag is enabled, the material property will be
compared against the pixel's alpha value to decide whether it should
get discarded.
Nodes can have either a transform matrix, or decomposed transform
properties, but never both. Using a union means we can store both
of those variants in the same piece of memory, using the existing
matrix boolean to figure out which one to use.
This reduces the size of the struct by 48 bytes (152 -> 104), which
ends up speeding up some model operations, I'm guessing due to the
CPU cache.