Refactor bmpcodec (mono#511)

Aims: - Make it simpler to read this 1000 line long function - Make it clearer what we're doing There are a number of discrete stages involved in decoding a bmp image - Reading the header - Reading the palette if any - Calculating the stride - Reading the scans - Converting the scans to a target pixel format if necessary This PR splits the core bmp decoding into several routines that clarify these stages and will make it easier for me to make future changes

Author: hughbe

src/bitmap-private.h

@@ -77,7 +77,7 @@ typedef struct {
 unsigned intwidth;
 unsigned intheight;
 intstride;
-intpixel_format;
+PixelFormatpixel_format;
 BYTE *scan0;
 UINT_PTRreserved;
 /* the rest of the structure isn't part of MS GDI+ definition */

src/bitmap.c

@@ -2097,16 +2097,12 @@ GdipBitmapGetPixel (GpBitmap *bitmap, INT x, INT y, ARGB *color)
 break;
 }
 case PixelFormat16bppARGB1555:
-case PixelFormat16bppRGB555: {
-WORD *scan = (WORD *) v;
-*color = gdip_convert_16bppRGB555_ToARGB (scan[x]);
+case PixelFormat16bppRGB555:
+*color = gdip_getpixel_16bppRGB555 (v, x);
 break;
-}
-case PixelFormat16bppRGB565: {
-WORD *scan = (WORD *) v;
-*color = gdip_convert_16bppRGB565_ToARGB (scan[x]);
+case PixelFormat16bppRGB565:
+*color = gdip_getpixel_16bppRGB565 (v, x);
 break;
-}
 default:
 return NotImplemented;
 }