Fix loading of RLE bitmaps in wxBMPHandler

- Fix handling of delta encoding that change vertical position
- Fix handling of end-of-line encodings that are given mid-line
- Removed unnecessary computation for linepos

Add regression tests for the bitmaps using RLE and loading which
previously didn't work correctly.

Closes https://github.com/wxWidgets/wxWidgets/pull/2590

Closes #19318.

tests/image/image.cpp

@@ -1355,6 +1355,11 @@ void ImageTestCase::BMPFlippingAndRLECompression()
     CompareBMPImage("image/horse_rle8.bmp", "image/horse_rle8_flipped.bmp");
 
     CompareBMPImage("image/horse_rle4.bmp", "image/horse_rle4_flipped.bmp");
+
+    CompareBMPImage("image/rle8-delta-320x240.bmp",
+                    "image/rle8-delta-320x240-expected.bmp");
+    CompareBMPImage("image/rle4-delta-320x240.bmp",
+                    "image/rle8-delta-320x240-expected.bmp");
 }