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)

src/imageformats/dds_p.h

@@ -2,6 +2,7 @@
     This file is part of the KDE project
     SPDX-FileCopyrightText: 2015 The Qt Company Ltd
     SPDX-FileCopyrightText: 2013 Ivan Komissarov
+    SPDX-FileCopyrightText: 2024 Mirco Miranda
 
     SPDX-License-Identifier: LGPL-2.1-only OR LGPL-3.0-only
 */
@@ -37,6 +38,15 @@ struct DDSPixelFormat
     quint32 aBitMask;
 };
 
+struct DDSHeaderDX10
+{
+    quint32 dxgiFormat = 0;
+    quint32 resourceDimension = 0;
+    quint32 miscFlag = 0;
+    quint32 arraySize = 0;
+    quint32 miscFlags2 = 0;
+};
+
 struct DDSHeader
 {
     enum DDSFlags {
@@ -84,15 +94,7 @@ struct DDSHeader
     quint32 caps3;
     quint32 caps4;
     quint32 reserved2;
-};
-
-struct DDSHeaderDX10
-{
-    quint32 dxgiFormat;
-    quint32 resourceDimension;
-    quint32 miscFlag;
-    quint32 arraySize;
-    quint32 reserved;
+    DDSHeaderDX10 header10;
 };
 
 class QDDSHandler : public QImageIOHandler
@@ -126,7 +128,6 @@ private:
 
     DDSHeader m_header;
     int m_format;
-    DDSHeaderDX10 m_header10;
     int m_currentImage;
     mutable ScanState m_scanState;
 };
@@ -136,8 +137,8 @@ class QDDSPlugin : public QImageIOPlugin
     Q_OBJECT
     Q_PLUGIN_METADATA(IID "org.qt-project.Qt.QImageIOHandlerFactoryInterface" FILE "dds.json")
 public:
-    Capabilities capabilities(QIODevice *device, const QByteArray &format) const;
-    QImageIOHandler *create(QIODevice *device, const QByteArray &format = QByteArray()) const;
+    Capabilities capabilities(QIODevice *device, const QByteArray &format) const override;
+    QImageIOHandler *create(QIODevice *device, const QByteArray &format = QByteArray()) const override;
 };
 
 #endif // QDDSHANDLER_H