Gecko vbuf backend: Gecko <= 1.9.2.9 (Firefox <= 3.6.9) crashes when we attempt to...

Gecko vbuf backend: Gecko <= 1.9.2.9 (Firefox <= 3.6.9) crashes when we attempt to retrieve table headers on certain table cells. We have no way of detecting when this is going to happen, so disable table headers completely for Gecko <= 1.9.2.9.
This is fixed in Gecko 1.9.2.10 (Firefox 3.6.10), which will be released soon.
Note that this crash also occurs in earlier Firefox 4 betas and nightlies. We don't protect against this for Firefox 4. Firefox 4 testers should use a recent nightly.
This required moving fillVBuf and fillTableCellInfo_IATable2 into the GeckoVBufBackend_t class. There is also a new method called versionSpecificInit which retrieves the version information and sets the flag to disable table headers.

Fixes #807.