用C语言显示PNG图片的解决办法
要求用纯C语言解决，不要GDI+

[解决办法]
设bkgR,G,B为背景当前像素RGB,取值范围0..255
pngR,G,B,A为PNG当前像素RGBA,取值范围0..255
则desRGB结果当前像素RGB为
desR=((int)(1.0-pngA/255.0)*bkgR+pngA/255.0*pngR)%256;
desG=((int)(1.0-pngA/255.0)*bkgG+pngA/255.0*pngG)%256;
desB=((int)(1.0-pngA/255.0)*bkgB+pngA/255.0*pngB)%256;