Fix wrong surface local coord for layer surface above an xwayland window

[seven-mile]

This PR fixes an incorrect selection rectangle issue that occurs when using slurp with a focused XWayland application.

While this represents a rare edge case, it consistently reproduces under the following technical circumstances:

• As a region selector, slurp requires users to maintain mouse button pressure until selection completion
• This action triggers special Drag-and-Drop focus retention logic, where pFoundWindow may be incorrectly set for layer surfaces (which slurp utilizes)
• When pFoundWindow references m_lastWindow from an XWayland instance with force_zero_scaling enabled, the special XWayland zero-scale handling branch activates
• The scaling effects intended for m_lastWindow erroneously apply to the current layer surface, corrupting surface-local coordinates

The solution prevents pFoundWindow assignment when encountering layer surfaces, which represents a logically sound approach.

https://github.com/user-attachments/assets/4f0269d7-ccff-4777-9970-8c2a5d7d33db