CoordGeom: Improve const correctness.

Also mark constant as float when it makes sense.
Subpic interfaces: Improve the const correctness.

The ABI isn't modified but the interfaces are for internal use anyway.
SubPicImpl: Minor cosmetics.
DX7 Subpic: Avoid resetting the static subpicture when possible.

Also use safer locking.
DX9 Subpic: Avoid resetting the static subpic and the subpic pool when possible.

Also use safer locking.
DX9 Subpic: Remove unused code.
Memory subpic: Mark some global variables static and minor cosmetics.
SubPicAllocators: Const correctness and minor cosmetics.
SubPicAllocators: Const correctness and minor cosmetics.
Subpic queues: Cosmetics.

Be less aggressive when invalidating the queue without threading while at it.
SubPic provider: Minor cosmetics.
Subtitle queue: Clearer display of the queue stats.

Show the start and end time of the queue without trying to normalize with current time.
Comment out some unused variable assignments.

Those can be useful if the functions are extended one day.
Don't force any background color for the Shockwave graph.

This commit fixes #1747.
Why "C:\tmp.srt"?
WebUI: Inform the web interface that no audio stream is available.
WebUI: Make use of new [hasaudio] info.
What the hell was intended here?!

Trying to make some sense of that code. Without much luck so far...