Improve rendering of start UI.

The background image is hidden when a team which has a custom logo image,
allowing the logo to appear without any distracting and mostly occluded
image behind.

This also better supports PNG logos with transparency.
Improve Bold Hearts team logo.
Merge pull request #3 from drewnoakes/master

New Bold Hearts logo, and cleaner display of team logos in start UI
Merge remote-tracking branch 'upstream/master'