Metal: Fixes for vertex binding state.

[squidbus]

• Replace vertex buffer tracking vectors with two arrays of the maximum binding length and a dirty binding bit-flag.
• Remove workaround to bind unused slots to null buffer as it is not needed anymore and was causing binding issues.
• Tidy up binding ranges in Metal.
  • Descriptor sets: Up to 8, from 0 to 7
  • Push constants: Up to 4, from 8 to 11
  • Vertex buffers: Up to 19, from 12 to 30
  • Reflected with two sets of constants: external maximum counts and the internal start index for each.
• Replaced buffer binding clamping with asserts as it will probably break something anyway if you hit it.

Fixes:

• Vertex bindings being lost if multiple setVertexBuffers happen between dirty state flushes.
• Vertex bindings being lost on partial bind.
• Vertex bindings other than the last partial bind being lost if the render encoder changes.