#20 Dont ignore ERASE_PAGE flag in single page transmissions

[Marton Balint]

It seems libzvbi ignores the ERASE_PAGE flag when multiple headers of the same page are following each other. The attached patch fixes it and the decoding of this file, in which subtitles are mixed up because the contents of the subtitle pages are not erased:
http://samples.ffmpeg.org/ffmpeg-bugs/trac/ticket2086/RBT_20100801_1835.ts