fix ofGet/SetBackgroundColor, replaces #1401

ofSetBackgroundColor has never worked properly, this fixes it
ofGetBackground is deprecated for ofGetBackgroundColor
ofBgColor removed completely, this will break old code but was really wrong
ofbBgClear removed for ofGetBackgroundAuto

also renderers now have to implement:

	startRender
	endRender

and windows need to call it in display.

windows don't need to set anymore the vieport, clear the bg...
now the renderers do that instead, before the window had to ask
the renderer if it had to clear the background and with which
color which was mixing the responsabilities of both classes and where
that info was stored
Merge branch 'master' into bugfix-ofGet/SetBackgroundColor
Merge branch 'master' into bugfix-ofGet/SetBackgroundColor

Conflicts:
	libs/openFrameworks/gl/ofShader.cpp
Merge pull request #3315 from arturoc/bugfix-ofGet/SetBackgroundColor

fix ofGet/SetBackgroundColor, replaces #1401