Improve blob extraction

[thaytan]

The CV1 touch controller LEDs can easily be only a bright pixel or two at around 1.2m distance from the camera. The current blob extraction code filters out such small blobs, which makes them occluded or missing as far as the pose search code is concerned.

[thaytan]

Increasing the quality of the blob extraction helps a lot. I've made some changes in my branch:

• Lower brightness cutoff for CV1. The CV1 camera seems to produce a wider range of pixel values and to ignore background noise better than the DK2. The LEDs are also smaller, so are often less bright at distance than the DK2 LEDs.
• Permit 2 pixel extents. I'd like to also allow 1 pixel extents, so we collect 1x2 blobs, but it produces too many 1x1 blobs as a consequence atm
• Allow more extents per line, and more blobs per frame. There are more LEDs to track now.

[thaytan]

The blob extraction is working well enough now - it's the matching code, and LED visibility prediction (and everything else) that needs improving now.