Windows: fix compiler warning [-Wstrict-aliasing]

In function ‘void innards(...)’:
src/drivers/GDI/Fl_GDI_Graphics_Driver_image.cxx:132:23:
dereferencing type-punned pointer will break strict-aliasing rules
  BITMAPINFO &bmi = *((BITMAPINFO*)bmibuffer);
                     ~^~~~~~~~~~~~~~~~~~~~~~~

Found with gcc 12 (MinGW cross compiler)

Also: use correct sizes (sizeof) rather than hardcoded values.

1 files changed, 22 insertions, 21 deletions

diff --git a/src/drivers/GDI/Fl_GDI_Graphics_Driver_image.cxx b/src/drivers/GDI/Fl_GDI_Graphics_Driver_image.cxx
index 356400da4..3a8e70689 100644
--- a/src/drivers/GDI/Fl_GDI_Graphics_Driver_image.cxx
+++ b/src/drivers/GDI/Fl_GDI_Graphics_Driver_image.cxx
@@ -1,7 +1,7 @@
 //
 // Windows image drawing code for the Fast Light Tool Kit (FLTK).
 //
-// Copyright 1998-2020 by Bill Spitzak and others.
+// Copyright 1998-2024 by Bill Spitzak and others.
 //
 // This library is free software. Distribution and use rights are outlined in
 // the file "COPYING" which should have been included with this file.  If this
@@ -128,43 +128,44 @@ static void innards(const uchar *buf, int X, int Y, int W, int H,
   if (w<=0 || h<=0) return;
   if (buf) buf += (x-X)*delta + (y-Y)*linedelta;
 
-  static U32 bmibuffer[256+12];
-  BITMAPINFO &bmi = *((BITMAPINFO*)bmibuffer);
-  if (!bmi.bmiHeader.biSize) {
-    bmi.bmiHeader.biSize = sizeof(bmi)-4; // does it use this to determine type?
-    bmi.bmiHeader.biPlanes = 1;
-    bmi.bmiHeader.biCompression = BI_RGB;
-    bmi.bmiHeader.biXPelsPerMeter = 0;
-    bmi.bmiHeader.biYPelsPerMeter = 0;
-    bmi.bmiHeader.biClrUsed = 0;
-    bmi.bmiHeader.biClrImportant = 0;
+  // bmibuffer: BITMAPINFOHEADER + 256 colors (RGBQUAD) + 1 (rounding effects ?)
+  static U32 bmibuffer[sizeof(BITMAPINFOHEADER)/4 + 257];
+  BITMAPINFO *bmi = (BITMAPINFO*)bmibuffer;
+  if (!bmi->bmiHeader.biSize) {
+    bmi->bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
+    bmi->bmiHeader.biPlanes = 1;
+    bmi->bmiHeader.biCompression = BI_RGB;
+    bmi->bmiHeader.biXPelsPerMeter = 0;
+    bmi->bmiHeader.biYPelsPerMeter = 0;
+    bmi->bmiHeader.biClrUsed = 0;
+    bmi->bmiHeader.biClrImportant = 0;
   }
 #if USE_COLORMAP
   if (indexed) {
     for (short i=0; i<256; i++) {
-      *((short*)(bmi.bmiColors)+i) = i;
+      *((short*)(bmi->bmiColors)+i) = i;
     }
   } else
 #endif
   if (depth<3) {
-    RGBQUAD *bmi_colors = &bmi.bmiColors[0];    // suppress warning (STR #3199)
+    RGBQUAD *bmi_colors = &(bmi->bmiColors[0]); // use pointer to suppress warning (STR #3199)
     for (int i=0; i<256; i++) {
-      bmi_colors[i].rgbBlue = (uchar)i;         // bmi.bmiColors[i]...
+      bmi_colors[i].rgbBlue = (uchar)i;     // = bmi->bmiColors[i]
       bmi_colors[i].rgbGreen = (uchar)i;
       bmi_colors[i].rgbRed = (uchar)i;
       bmi_colors[i].rgbReserved = (uchar)0; // must be zero
     }
   }
-  bmi.bmiHeader.biWidth = w;
+  bmi->bmiHeader.biWidth = w;
 #if USE_COLORMAP
-  bmi.bmiHeader.biBitCount = indexed ? 8 : depth*8;
+  bmi->bmiHeader.biBitCount = indexed ? 8 : depth*8;
   int pixelsize = indexed ? 1 : depth;
 #else
-  bmi.bmiHeader.biBitCount = depth*8;
+  bmi->bmiHeader.biBitCount = depth*8;
   int pixelsize = depth;
 #endif
   if (depth==2) { // special case: gray with alpha
-    bmi.bmiHeader.biBitCount = 32;
+    bmi->bmiHeader.biBitCount = 32;
     pixelsize = 4;
   }
   int linesize = (pixelsize*w+3)&~3;
@@ -185,7 +186,7 @@ static void innards(const uchar *buf, int X, int Y, int W, int H,
       buffer = new U32[(size + 3) / 4];
     }
   }
-  bmi.bmiHeader.biHeight = blocking;
+  bmi->bmiHeader.biHeight = blocking;
   static U32* line_buffer;
   if (!buf) {
     int size = W*delta;
@@ -259,7 +260,7 @@ static void innards(const uchar *buf, int X, int Y, int W, int H,
       // does not do the expected job, whereas StretchDIBits does it.
       StretchDIBits(gc, x, y+j-k, w, k, 0, 0, w, k,
                     (LPSTR)((uchar*)buffer+(blocking-k)*linesize),
-                    &bmi,
+                    bmi,
 #if USE_COLORMAP
                     indexed ? DIB_PAL_COLORS : DIB_RGB_COLORS
 #else
@@ -273,7 +274,7 @@ static void innards(const uchar *buf, int X, int Y, int W, int H,
     else {
       SetDIBitsToDevice(gc, x, y+j-k, w, k, 0, 0, 0, k,
                         (LPSTR)((uchar*)buffer+(blocking-k)*linesize),
-                        &bmi,
+                        bmi,
 #if USE_COLORMAP
                         indexed ? DIB_PAL_COLORS : DIB_RGB_COLORS
 #else