Internal filters: VP9 can now be enabled/disable from option page.
Internal filters: HEVC can now be enabled/disable from option page.
Remove logically dead code.

Fixes CID #965942 #1109592.
Fix uninitialized scalar variable.

Fixes CID #966220 #966223.
Fix an incorrect sizeof.

Fixes CID #966192.
Check return value.

Fixes CID #1109591, #1084350
Fix uninitialized class members.

Fixes CID #966364, #966344, #966315, #966315, #966347, #966319, #966314,
 #966313, #966280
Fix the /W4 C4245 warnings: signed/unsigned mismatch.
AppSettings: Move some backward compatibility code to UpdateSettings()