Improve trackball rotation mode by using a single axis

Currently, trackball rotation sequentially applies rotation across x axis and y axis, which produces weird results when mouse pointer is moving diagonally:

gfycat.com/WealthyQuaintBovine

If rotation is combined properly, this problem does not happen:

gfycat.com/IndolentZigzagGroundbeetle

Below is a complete patch to implement this change:

developer.blender.org/differential/diff/8971/