GSP: Cleaned up command buffer decoding.

GSP: Cleaned up code and added additional comments.

GSP: Removed unnecessary TODO comment.

GSP: Changed u32 iterators in TriggerCmdReqQueue to unsigned.
Thread: Added more descriptive comment to WaitCurrentThread.
GPU: Updated horizontal sync line counter to use framebuffer height.
GPU: Updated g_last_ticks variable to be more descriptive (represents CPU tick count of last vertical line).