libavformat/mtv: Check for min header size first

Abort immediately if we are not getting enough
data to extract the required fields.

Signed-off-by: Reynaldo H. Verdejo Pinochet <r.verdejo@sisa.samsung.com>
libavformat/mtv: donwgrade probe score for < 512

MTV has a 512 bytes header. We should be able to
parse required fiels from the first 57 (rest is
padding) but if we haven't seen enough data then
go with SCORE_EXTENSION.

Signed-off-by: Reynaldo H. Verdejo Pinochet <r.verdejo@sisa.samsung.com>
libavformat/mtv: make clear we assume bpp is always 16

All samples in the wild are RGB565/555 and we are already
acting on this assumption when pushing out the video frames,
so if we get anything != than 16 for bpp we just override
this value for doing any calculations making our approach
consistent.

Also avoid repeatedly shifting bpp.

Signed-off-by: Reynaldo H. Verdejo Pinochet <r.verdejo@sisa.samsung.com>
libavformat/mtv: add missing MTV_ prefix to macro

Signed-off-by: Reynaldo H. Verdejo Pinochet <r.verdejo@sisa.samsung.com>