[wires-grids] CellContextUtilities.makeCellRenderContext() wrong for scrolled grids

The cellY returned by CellContextUtilities.makeCellRenderContext(..) is wrong when the grid is scrolled. This is caused by the calculation for the cell's absolute Y co-ordinate being based on the header's Y co-ordainte. The cell's Y co-ordinate should always be calculated from the grid's computed location Y co-ordinate and not the header as the header floats when a grid is scrolled.

Acceptance criteria

Perform an action that invokes the affected code with the grid scrolled (as at writing this is limited to the keyboard launch of context menus in the DMN editor). This can be accomplished with:

• Create a DMN file
• Add a Decision node
• Set the Decision node's Expression to a Decision Table
• Add lots of rows (so that the grid can be scolled)
• Scroll the grid (so row 1 is not at the top)
• Select a cell (for example row 3, column 2)
• Press ctrl+space to launch the context menu
• The context menu's top-left should be in the middle of the cell
• Prepare enough columns to be able to scroll horizontally
• Try to edit header cells
• Try to insert column