Replace setPixelColor with scanLine() or bits()

[pklampros]

From the qt docs for setPixelColor:

Warning: This function is expensive due to the call of the internal detach() function called within; if performance is a concern, we recommend the use of scanLine() or bits() to access pixel data directly.

As found when filling pointmap pixels:

depthmapX/depthmapX/views/glview/glpointmap.cpp

Line 67 in b943241

data.setPixelColor(x, y, qRgb(colour.redb(),colour.greenb(),colour.blueb()));