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

---

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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/JpegToFramebufferConverter.cpp

@@ -20,60 +20,37 @@ namespace {
 // The draw callback receives this via pDraw->pUser (set by setUserPointer()).
 // The file I/O callbacks receive the FsFile* via pFile->fHandle (set by jpegOpen()).
 struct JpegContext {
-  GfxRenderer* renderer;
-  const RenderConfig* config;
-  int screenWidth;
-  int screenHeight;
+  GfxRenderer* renderer{nullptr};
+  const RenderConfig* config{nullptr};
+  int screenWidth{0};
+  int screenHeight{0};
 
   // Source dimensions after JPEGDEC's built-in scaling
-  int scaledSrcWidth;
-  int scaledSrcHeight;
+  int scaledSrcWidth{0};
+  int scaledSrcHeight{0};
 
   // Final output dimensions
-  int dstWidth;
-  int dstHeight;
+  int dstWidth{0};
+  int dstHeight{0};
 
   // Fine scale in 16.16 fixed-point (ESP32-C3 has no FPU)
-  int32_t fineScaleFP;  // src -> dst mapping
-  int32_t invScaleFP;   // dst -> src mapping
+  int32_t fineScaleFP{1 << 16};  // src -> dst mapping
+  int32_t invScaleFP{1 << 16};   // dst -> src mapping
 
   PixelCache cache;
-  bool caching;
+  bool caching{false};
 
   // See PngContext for the rationale: monochromeOutput requests a 1-bit Atkinson dither
   // emitting only 0/3 so the BW DirectPixelWriter (`pixelValue < 3` rule) maps cleanly.
-  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
 
-  JpegContext()
-      : renderer(nullptr),
-        config(nullptr),
-        screenWidth(0),
-        screenHeight(0),
-        scaledSrcWidth(0),
-        scaledSrcHeight(0),
-        dstWidth(0),
-        dstHeight(0),
-        fineScaleFP(1 << 16),
-        invScaleFP(1 << 16),
-        caching(false),
-        oneBitDitherRow(-1),
-        atkinson1BitDitherer(nullptr)
-#ifdef ENABLE_IMAGE_DITHERING_EXTENSION
-        ,
-        currentDitherRow(-1),
-        atkinsonDitherer(nullptr),
-        diffusedBayerDitherer(nullptr)
-#endif
-  {
-  }
-
   ~JpegContext() {
     delete atkinson1BitDitherer;
 #ifdef ENABLE_IMAGE_DITHERING_EXTENSION