ximgproc: EdgeDrawing must not modify the caller's input when Sigma < 1.0

[purehol]

Root cause

When Params::Sigma < 1.0, detectEdges() set smoothImage = srcImage, aliasing
the caller's input Mat. detectEllipses() then runs an in-place GaussianBlur
into smoothImage (the PFmode path in detectEdges() does the same), silently
overwriting the user's input image and any later reads of it.

Fix

Clone the image so the input stays read-only, matching the reference implementation
ED_Lib (which always blurs into a separately allocated buffer). Cost is one
width*height copy when Sigma < 1.0; correctness takes priority.

Verification

Added TEST_F(ximgproc_ED, detectEllipsesDoesNotModifyInput): with Sigma = 0.0
it asserts the input Mat is unchanged after detectEdges() + detectEllipses().
Fails before this change (1916 input pixels modified) and passes after it;
existing *ED* tests still pass.

Fixes #4203

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