Remove offset argument from TrackedRenderPass::set_index_buffer

[akimakinai]

Objective

• offset argument is misleading as the argument is not actually passed to wgpu (used only for memoization and logging). BufferSlice already contains an offset.
  • wgpu's set_index_buffer sets the offset according to BufferSlice::offset
• TrackedRenderPass::set_vertex_buffer was made aware of slice size (Make TrackedRenderPass::set_vertex_buffer aware of slice size #14916) but missed set_index_buffer counterpart

Solution

• Removed offset argument from TrackedRenderPass::set_index_buffer
• Apply fix from Make TrackedRenderPass::set_vertex_buffer aware of slice size #14916 to TrackedRenderPass::is_index_buffer_set
• Cleanup code by using the newly added BufferSlice getters Add getters and From implementations to BufferSlice. gfx-rs/wgpu#7148

Testing

• Ran a few examples