use a u64 for MeshPipelineKey

[mockersf]

Objective

• MeshPipelineKey use some bits for two things
• First commit in this PR adds an assertion that doesn't work currently on main
• This leads to some mesh topology not working anymore, for example LineStrip
• With examples lines, there should be two groups of lines, the blue one doesn't display currently

Solution

• Change the MeshPipelineKey to be backed by a u64 instead, to have enough bits

[pcwalton]

Do you know if there's any perf impact from this?

Anyway this is fine since it fixes bugs.

[james7132]

Increasing the storage size isn't ideal when we could localize the extension to the places where we're doing those bitshift operations. It's not a surefire way to avoid the issue, but the primary areas where those shifts are done are already encapsulated in functions on MeshPipelineKey itself (i.e. from_primitive_toplogy)

[mockersf]

Not sure I understand what you mean? The bits should still be reserved for topology, even outside of from_primitive_toplogy

The bits from the base key are reserved from the end, while the bits from the key are reserved from the start, they should not overlap

[james7132]

Oh I was misreading the code and thought this problem was a result of bitshifting erasing some of the bits, disregard my prior comment.

[mockersf]

also fixed morph bit, used les magic numbers, and updated comments