refactor: Use default member initializers for JpegContext and PngContext (#1435)

**What is the goal of this PR?**

Replace verbose constructor initializer lists with in-class default
member initializers in JpegContext and PngContext

---

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? _**NO**_

lib/Epub/Epub/converters/PngToFramebufferConverter.cpp

@@ -20,62 +20,38 @@ namespace {
 // The draw callback receives this via pDraw->pUser (set by png.decode()).
 // The file I/O callbacks receive the FsFile* via pFile->fHandle (set by pngOpen()).
 struct PngContext {
-  GfxRenderer* renderer;
-  const RenderConfig* config;
-  int screenWidth;
-  int screenHeight;
+  GfxRenderer* renderer{nullptr};
+  const RenderConfig* config{nullptr};
+  int screenWidth{0};
+  int screenHeight{0};
 
   // Scaling state
-  float scale;
-  int srcWidth;
-  int srcHeight;
-  int dstWidth;
-  int dstHeight;
-  int lastDstY;  // Track last rendered destination Y to avoid duplicates
+  float scale{1.f};
+  int srcWidth{0};
+  int srcHeight{0};
+  int dstWidth{0};
+  int dstHeight{0};
+  int lastDstY{-1};  // Track last rendered destination Y to avoid duplicates
 
   PixelCache cache;
-  bool caching;
+  bool caching{false};
 
-  uint8_t* grayLineBuffer;
+  uint8_t* grayLineBuffer{nullptr};
 
   // When the caller requests monochrome output (RenderConfig::monochromeOutput),
   // we run a proper 1-bit Atkinson dither (matching PngToBmpConverter's BW path)
   // and emit only values 0 or 3, which round-trip cleanly through the BW writer's
   // `pixelValue < 3` rule. The 4-level dither path collapses mid-grays to solid
   // black under that rule.
-  int oneBitDitherRow;
-  Atkinson1BitDitherer* atkinson1BitDitherer;
+  int oneBitDitherRow{-1};
+  Atkinson1BitDitherer* atkinson1BitDitherer{nullptr};
 
 #ifdef ENABLE_IMAGE_DITHERING_EXTENSION
-  int currentDitherRow;
-  AtkinsonDitherer* atkinsonDitherer;
-  DiffusedBayerDitherer* diffusedBayerDitherer;
+  int currentDitherRow{-1};
+  AtkinsonDitherer* atkinsonDitherer{nullptr};
+  DiffusedBayerDitherer* diffusedBayerDitherer{nullptr};
 #endif
 
-  PngContext()
-      : renderer(nullptr),
-        config(nullptr),
-        screenWidth(0),
-        screenHeight(0),
-        scale(1.0f),
-        srcWidth(0),
-        srcHeight(0),
-        dstWidth(0),
-        dstHeight(0),
-        lastDstY(-1),
-        caching(false),
-        grayLineBuffer(nullptr),
-        oneBitDitherRow(-1),
-        atkinson1BitDitherer(nullptr)
-#ifdef ENABLE_IMAGE_DITHERING_EXTENSION
-        ,
-        currentDitherRow(-1),
-        atkinsonDitherer(nullptr),
-        diffusedBayerDitherer(nullptr)
-#endif
-  {
-  }
-
   ~PngContext() {
     delete atkinson1BitDitherer;
 #ifdef ENABLE_IMAGE_DITHERING_EXTENSION