Fix DND coordinates on Windows
This MR fixes some of the issues with DND handling on Windows, specifically those issues in relation to pointer input. It's comprised of two commits:
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The first commit makes the DND indicator surface transparent w.r.t pointer input. This is required in order to avoid making the DND indicator itself a target of the DND operation, which otherwise would render the DND non-functional.
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The second commit fixes some mismatches where screen coordinates are passed as surface-relative coordinates and viceversa.
A very useful test for this MR is tests/testdnd.
Edited by Luca Bacci