[AD] Additional functionality for load_bmp

["Chris Jones" <cj@xxxxxxxxxx>]

I've had a few complaints from some people using my software that it won't
import some BMP files. I did a little investigation and it turns out these
files use the BI_BITFIELDS compression type, which Allegro doesn't support.

The attached patch adds support for 15-bit, 16-bit and 32-bit BMP files of
this type (according to MSDN, I think these are the only colour depths that
use this type). It works with the two files I have to test it with, so if
you approve please do add it to allegro.

Patch is against BMP.C from Allegro 4.1.12

Cheers,
Chris

276a277,330
> static void read_bitfields_image(PACKFILE *f, BITMAP *bmp, AL_CONST BITMAPINFOHEADER *infoheader)
> {
> 	int k, i;
> 	int bpp;
> 	int bytesPerPixel;
> 	int red, grn, blu;
> 	unsigned long buffer;
> 
> 	bpp = bitmap_color_depth(bmp);
> 
> 	if (bpp == 15)
> 		bytesPerPixel = 2;
> 	else
> 		bytesPerPixel = bpp / 8;
> 
> 	for (i=0; i<(int)infoheader->biHeight; i++) {
> 		for (k=0; k<(int)infoheader->biWidth; k++) {
> 			pack_fread(&buffer, bytesPerPixel, f);
> 
> 			if (bpp == 15)
> 			{
> 				red = (buffer >> 10) & 0x1f;
> 				grn = (buffer >> 5) & 0x1f;
> 				blu = (buffer) & 0x1f;
> 				buffer = (red << _rgb_r_shift_15) |
> 						(grn << _rgb_g_shift_15) |
> 						(blu << _rgb_b_shift_15);
> 			}
> 			else if (bpp == 16)
> 			{
> 				red = (buffer >> 11) & 0x1f;
> 				grn = (buffer >> 5) & 0x3f;
> 				blu = (buffer) & 0x1f;
> 				buffer = (red << _rgb_r_shift_16) |
> 						(grn << _rgb_g_shift_16) |
> 						(blu << _rgb_b_shift_16);
> 			}
> 			else
> 			{
> 				red = (buffer >> 16) & 0xff;
> 				grn = (buffer >> 8) & 0xff;
> 				blu = (buffer) & 0xff;
> 				buffer = (red << _rgb_r_shift_32) |
> 						(grn << _rgb_g_shift_32) |
> 						(blu << _rgb_b_shift_32);
> 
> 			}
> 
> 			memcpy(&bmp->line[(infoheader->biHeight - i) - 1][k * bytesPerPixel], &buffer, bytesPerPixel);
> 		}
> 	}
> }
> 
> 
502c556,558
<       read_bmicolors(ncol, pal, f, 1);
---
> 
> 	  if (infoheader.biCompression != BI_BITFIELDS)
> 		read_bmicolors(ncol, pal, f, 1);
511c567,569
<       read_bmicolors(ncol, pal, f, 0);
---
> 
> 	  if (infoheader.biCompression != BI_BITFIELDS)
> 		read_bmicolors(ncol, pal, f, 0);
519a578,579
>    else if (infoheader.biBitCount == 16)
> 	  bpp = 16;
522a583,603
>    if (infoheader.biCompression == BI_BITFIELDS)
>    {
> 		unsigned long redMask = pack_igetl(f);
> 		unsigned long grnMask = pack_igetl(f);
> 		unsigned long bluMask = pack_igetl(f);
> 
> 		if ((bluMask == 0x001f) && (redMask == 0x7C00))
> 			bpp = 15;
> 		else if ((bluMask == 0x001f) && (redMask == 0xF800))
> 			bpp = 16;
> 		else if ((bluMask == 0x0000FF) && (redMask == 0xFF0000))
> 			bpp = 32;
> 		else
> 		{
> 			// Unrecognised bit masks/depth
> 			pack_fclose(f);
> 			return NULL;
> 		}
>    }
> 
> 
545a627,630
> 
> 	  case BI_BITFIELDS:
> 		read_bitfields_image(f, bmp, &infoheader);
> 		break;