DVBSub/PGSSub: Add more error checking when parsing composition object.

Fixes CID #971508.
Initialize a variable.

Fixes CID #1111779.
Remove some dead code.

Fixes CID #965946.
Improve the Get(Cur)DispMode and SetDispMode functions.

Fixes CID #966222.
VSFilter: Use the "secure" version of the string functions when possible.
Fix/silence some warnings (mostly "possible loss of data").
Update LAV Filters to 0.58.2-95-f326e37 (custom build based on 0.58.2-90-602b085).

Important change: Fix a crash when opening RealMedia streaming (ticket #3342).