optimize imagery layer alpha=0
Merge remote-tracking branch 'agi/master' into optimizeImageryAlpha
allow tiles to be rendered before zero alpha layers are loaded, replace epsilon check with compare to 0.0
Update CHANGES.md.
Merge remote-tracking branch 'origin/master' into optimizeImageryAlpha

Conflicts:
	CHANGES.md
Merge pull request #576 from NICTA/optimizeImageryAlpha

optimize imagery layer alpha=0