scrollwheel zooming with button pressed, offset not calculated on scroll event

In v0.48.4 r9939,
it's possible to zoom in and out with the scrollwheel
while drawing/moving a shape/path or rotating a guide.

It helps with precision when positioning an object while it's being drawn or moved (scaling and rotating objects or moving guides doesn't work currently).

Problem:
As one scrolls to zoom in and out,
the object being drawn/moved sometimes shifts away from the cursor,
and when the cursor is moved (a mouse drag event),
the object returns to where the mouse was pressed on it.

Solution:
It is caused by the offset distance between the mouse coordinates and the bottom left corner of the page only gets calculated when the mouse is pressed, it should also be calculated in the scroll event code, if a button is pressed and the scroll wheel is turned.

Reproduce:
preferences > scrolling > [checked] mouse wheel zooms by default

Zooming with the middle mouse button pressed:
(it's probably not done very often but it demonstrates the offset problem)
1. move the cursor to the top right corner of the page
2. hold down the middle mouse button (the scroll wheel)
3. try not to move the mouse and turn the scrollwheel,
if the mouse wasn't moved while scrolling (it's not easy, 1px is enough),
then the page zooms normally.
4. now move the mouse (the offset is updated on mouse drag),
and the page snaps to maintain the same distance between the mouse and bottom left corner of the page.

Drawing a shape:
1. choose the rectangle tool
2. hold down the left mouse button to start drawing a rectangle
3. move the cursor so that the rectangle has some width and height
4. scroll the mouse wheel to zoom in and out (it does not have to be done quickly)
5. the corner of the rectangle that is under the cursor sometimes shifts away from the cursor
6. if the mouse isn't moved (not easy) and one keeps zooming in and out
then the drawing corner of the rectangle stays shifted from the cursor
7. now move the cursor, the rectangles corner snaps back under the cursor.

Moving a node:
1. choose the bezier path tool
2. draw 2 nodes on the page
3. choose the edit paths and nodes tool
4. hold down the left mouse button on one of the nodes and move it slightly.
5. scroll up, the second node becomes selected,
6. move the mouse, the second node shifts to the bottom left corner, the path between them shows the offset distance between the mouse and bottom left corner that was set when the first node was moved slightly.

This zooming feature seams to have been removed in the development versions iv'e tried since r12830, maybe it was removed because of the offset problem.

tested on linux mint 15 cinnamon 32bit