Metal: fix vertex attribute's conversion lost after changing buffer binding.

After vertex buffer's attribute is converted and stored in conversion
buffer. Binding the same attribute to another buffer, then binding it
back to previous buffer will result in previous conversion
information lost. The conversion method would skip the conversion due to
buffer's content hadn't been changed, however it didn't reuse the old
conversion result.

This CL also changed the way binding offset is used in Metal backend.
- Previous, the offset would be assigned to the offset field of
  MTLVertexAttributeDescriptor, then the buffer would simply be bound to
  the command encoder with offset=0
  i.e. setVertexBuffer(buffer, index, 0)
- However this approach has several disadvantages. Since Metal doesn't
  allow MTLVertexAttributeDescriptor's offset to be larger than the
  vertex attribute's stride, the old approach would force the back-end
  to convert the attribute and store in conversion buffer.

New approach:
- MTLVertexAttributeDescriptor's offset will be zero. The offset will be
  used to bind the buffer itself to the render command encoder.
  i.e. setVertexBuffer(buffer, index, offset)
  This way the "offset <= stride" restriction no longer exists. The only
  restriction is the offset must be multiple of attribute's size.

Added 3 new tests:
- SimpleStateChangeTest.RebindTranslatedAttribute
- VertexAttributeTest.DrawWithLargeBufferOffset
- VertexAttributeTest.DrawWithLargeBufferOffsetAndLessComponents

Bug: angleproject:2634
Change-Id: I6c2fa8091436e4a24405d791f86d17d97df02d64
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1940009
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>