scrna plot: zoom and reset issue

[xzhou82]

after panning, reset doesn't recenter

zooming should only occur when cursor hover over map. shouldn't occur when cursor is outside of the map. also please verify if the zoom is opposite

in gene exp overlay hard to see dark cells, is it rendering cells by the ascending order of gene exp value? if not the highly expressed cells are covered by blank ones

[xzhou82]

@airenzp issue reoccurred in master. at gdc plot, plot.plane is undefined at line 396 and 406, causing zoom and reset button to break