Fix alpha pre-multiplication.

The alpha pre-multiplication was applied to an aggregate value an not to the color value. This could cause wrong colors for images with an alpha channel.
Fixes #47.

converter.go

@@ -158,20 +158,20 @@ func resizeNRGBA(in *image.NRGBA, out *image.RGBA, scale float64, coeffs []int16
 						xi = 0
 					}
 
-					rgba[0] += int32(coeff) * int32(row[xi+0])
-					rgba[1] += int32(coeff) * int32(row[xi+1])
-					rgba[2] += int32(coeff) * int32(row[xi+2])
-					rgba[3] += int32(coeff) * int32(row[xi+3])
-					sum += int32(coeff)
-
 					// Forward alpha-premultiplication
 					a := int32(row[xi+3])
-					rgba[0] *= a
-					rgba[0] /= 0xff
-					rgba[1] *= a
-					rgba[1] /= 0xff
-					rgba[2] *= a
-					rgba[2] /= 0xff
+					r := int32(row[xi+0]) * a
+					r /= 0xff
+					g := int32(row[xi+1]) * a
+					g /= 0xff
+					b := int32(row[xi+2]) * a
+					b /= 0xff
+
+					rgba[0] += int32(coeff) * r
+					rgba[1] += int32(coeff) * g
+					rgba[2] += int32(coeff) * b
+					rgba[3] += int32(coeff) * a
+					sum += int32(coeff)
 				}
 			}
 
@@ -259,20 +259,20 @@ func resizeNRGBA64(in *image.NRGBA64, out *image.RGBA64, scale float64, coeffs [
 						xi = 0
 					}
 
-					rgba[0] += int64(coeff) * int64(uint16(row[xi+0])<<8|uint16(row[xi+1]))
-					rgba[1] += int64(coeff) * int64(uint16(row[xi+2])<<8|uint16(row[xi+3]))
-					rgba[2] += int64(coeff) * int64(uint16(row[xi+4])<<8|uint16(row[xi+5]))
-					rgba[3] += int64(coeff) * int64(uint16(row[xi+6])<<8|uint16(row[xi+7]))
-					sum += int64(coeff)
-
 					// Forward alpha-premultiplication
 					a := int64(uint16(row[xi+6])<<8 | uint16(row[xi+7]))
-					rgba[0] *= a
-					rgba[0] /= 0xffff
-					rgba[1] *= a
-					rgba[1] /= 0xffff
-					rgba[2] *= a
-					rgba[2] /= 0xffff
+					r := int64(uint16(row[xi+0])<<8|uint16(row[xi+1])) * a
+					r /= 0xffff
+					g := int64(uint16(row[xi+2])<<8|uint16(row[xi+3])) * a
+					g /= 0xffff
+					b := int64(uint16(row[xi+4])<<8|uint16(row[xi+5])) * a
+					b /= 0xffff
+
+					rgba[0] += int64(coeff) * r
+					rgba[1] += int64(coeff) * g
+					rgba[2] += int64(coeff) * b
+					rgba[3] += int64(coeff) * a
+					sum += int64(coeff)
 				}
 			}