DDS: improved read/write support

The following changes have been made:

- Improved writing speed by using scanLine() instead of pixel()
- Optimized memory usage on writing by using ScanlineConverter class
- Added native write support for RGBA32FPx4, RGBA16FPx4, RGB8, Grayscale8 and Indexed8 uncompressed formats
- Grayscale DDS without alpha are loaded in a Grayscale8 image
- Fixed warnings about wrong PITCH reported by GIMP on DDSs saved by this plugin
- Initial support for loading DX10 formats (R16F, RG16F, RGBA16F, RGBPreMulA16F, R32F, RG32F, RGBA32F, RGBPreMulA32F)
- Fixed alignment issues and A8P8 format support of the attached images*

Tested using GIMP and [NVIDIA Texture Tools](https://developer.nvidia.com/texture-tools-exporter) plugin for Photoshop.

(*) The following images (taken from [here](https://github.com/walbourn/directxtexmedia)) cannot be added to read tests due to license issue:
[test8_DWORD.dds](/uploads/449b5a0d886aaf6764af554fe38e2b09/test8_DWORD.dds)
[dx5_logo.dds](/uploads/6f5f27df752890b227ef07e0195435d4/dx5_logo.dds)
[test888_DWORD.dds](/uploads/c8bc355c5749cf203d47e0b3073ad419/test888_DWORD.dds)

autotests/CMakeLists.txt

@@ -78,7 +78,7 @@ kimageformats_read_tests(
 
 if(KIMAGEFORMATS_DDS)
     kimageformats_read_tests(dds)
-    kimageformats_write_tests(dds)
+    kimageformats_write_tests(dds-nodatacheck-lossless)
 endif()
 
 if (KF6Archive_FOUND)