Blitting image with alpha channel doesn't seem blend alpha

Loading a transparent PNG then blitting it seems to always have a light background instead of blending it with what's already there.

E.g. with this image:

animation { width = 1000, height = 400, length = 1, framerate = 60 }

gitLogo = image.load("Git-Logo-2Color.png")
nearlyblack = solid(hex("#202020"))

function render()
  clear(nearlyblack)
  blit(gitLogo, 0, 0)
end

The issue might be that Cairo's ARGB32 format expects pre-multiplied alpha. However, I'm not sure this is accounted in the image loading function as PNG requires non-pre-multiplied alpha:

pan/src/panpkg/api.nim

Lines 158 to 168 in 6289a27

    
           # convert from RGBA to ARGB because cairo <3 
        
           for y in 0..<height: 
        
             for x in 0..<width: 
        
               let 
        
                 r = channels * (x + y * width) 
        
                 g = r + 1 
        
                 b = g + 1 
        
                 a = b + 1 
        
               (img[r], img[g], img[b], img[a]) = 
        
                 when cpuEndian == bigEndian: (img[a], img[r], img[g], img[b]) 
        
                 else: (img[b], img[g], img[r], img[a])

	# convert from RGBA to ARGB because cairo <3
	for y in 0..<height:
	for x in 0..<width:
	let
	r = channels * (x + y * width)
	g = r + 1
	b = g + 1
	a = b + 1
	(img[r], img[g], img[b], img[a]) =
	when cpuEndian == bigEndian: (img[a], img[r], img[g], img[b])
	else: (img[b], img[g], img[r], img[a])