Minor refactor
1. Add VSFilter compact RGB correction and its REG option. The REG option(RGB_CORRECTION@General) default to "auto", with which correction would be applied if 1.1 the subtitle is ASS/SSA/SRT and 1.2 the consumer report a TV.709 YUV matrix and 1.3 a TV.601 is expected to apply to the subtitle (in VSFilter) for now. To disable it set the REG option RGB_CORRECTION@General to "never". 2. Option "yuvMatrix" now reports a) HDMVs: YUV matrix used; b) ASS/SSA/SRT: YUV matrix that is expected (None if RGB correction has been done); c) others: None.
BugFix: wrong class used for dynamic cast
BugFix: NULL pos causes a crash
BugFix: failed to reset subtitle when output/subtitle target rectangle changed
BugFix: external subtitle auto-loading