[Win32] Improve size calculation for cropped and scaled image drawing

[HeikoKlare]

When drawing cropped and scaled images, the calculation for the source rectangle to draw is quite error prone:

• It does not distinguish between different scale factor in X and Y direction, leading to large rounding errors if the extent in one directions is highly different from the extent in the other direction
• It does not apply rounding that is consistent to the scaling done by the Image class, thus leading to differently rounded sizes when scaling an image in the Image class and drawing that same image in the GC
• The "error correction" to deal with rounding at fractional scale factors is too restrictive, in particular when the scale factor is less than 1

This change reimplements the source rectangle calculation as follows:

• It treats the scale factors for both axes independently
• It applies the same rounding method to the rectangle extents than done by Image scaling implementation
• It rounds up the scale factor when checking for the allowed size error on fractional scaling, such that a scale factor less than 1 still allows for an error of 1 in size.

This is currently a draft for the sake of visibility that may address eclipse-gef/gef-classic#1146 and other not-yet-properly-documented issues (as sketched in the description above).

[github-actions]

Test Results (win32)

   33 files  ±0     33 suites  ±0   4m 30s ⏱️ +14s
4 718 tests ±0  4 642 ✅ ±0  76 💤 ±0  0 ❌ ±0 
1 247 runs  ±0  1 223 ✅ ±0  24 💤 ±0  0 ❌ ±0 

Results for commit 31181b5. ± Comparison against base commit 28ddc85.