WarpBlt from 16bpp to 32bpp

>   Hello,
>
>   I think that this is a known issue, but can't remember when was the
> last time we discussed this.  So, I'm bringing it up in a context I
> encountered.
>
>   Here is a sample code to illustate the problem:
>
> -------------------
> f _ (Form extent: 50@50 depth: 16) fillColor: Color green.
> g _ Form extent: 100@100 depth: 32.
>
> (WarpBlt current toForm: g)
>   sourceForm: f;
>   cellSize: 2;
>   combinationRule: Form over;
>   sourceQuad: (f boundingBox) innerCorners destRect: g boundingBox;
>   warpBits.
>
> "g fixAlpha."
> g colorAt: 50@50  
> -------------------
>
>   First you fill a 16-bit form with green, and then warp blt it onto a
> 32-bit Form with the "over" rule.  You'd expect to get a green-colored
> form as a result, right?
>
>   One of the problem is that the alpha channel is set to 0 so it
> results in "Color transparent".  You would imagine to fix it by "g
> fixAlpha" above.
>
>   However, if you uncomment the line and execute, you get "(Color r:
> 0.94 g: 0.0 b: 0.0)" as the result.  This is wrong.
>
>   A part of BitBltSimulation>>warpPickSmoothPixels: nPixels
>                               xDeltah: xDeltah yDeltah: yDeltah
>                               xDeltav: xDeltav yDeltav: yDeltav
>                               sourceMap: sourceMap
>                               smoothing: n
>                               dstShiftInc: dstShiftInc
>
> says:
>
> "normalize rgba sums"
> nPix = 4 "Try to avoid divides for most common n"
> ifTrue:[r _ r >> 2. g _ g >> 2. b _ b >> 2. a _ a >> 2]
> ifFalse:[ r _ r // nPix. g _ g // nPix. b _ b // nPix. a _ a // nPix].
> rgb _ (a << 24) + (r << 16) + (g << 8) + b.
>
> "map the pixel"
> rgb = 0 ifTrue: [
> "only generate zero if pixel is really transparent"
> (r + g + b + a) > 0 ifTrue: [rgb _ 1]].
> rgb _ self mapPixel: rgb flags: cmFlags.
>
> With the first assignment, the variable "rgb" holds a 32-bit pixel
> value.  We know the destination is also 32 bit, so we even doesn't
> have to map the pixel value.  However, the invocation of
> #mapPixel:flags: takes the rgb value as if it is 16-bit and mask and
> shift again.
>
>   The right thing is to map the averaged rgb value back to the pixel
> value in the source depth and then map the color.  For a practical
> need, we can skip mapPixel:flags: if the dest is 32 and there is no
> color map specified.
>
>   And, the workaround I use is to use cellSize: 1 for this... But here
> is another issue, actually.  WarpBlt>>cellSize: says:
>
> cellSize: s
> cellSize _ s.
> cellSize = 1 ifTrue: [^ self].
> colorMap _ Color colorMapIfNeededFrom: 32 to: destForm depth.
>
> so it trys to install a color map from 32-bit regardless the actuall
> depth of the source.  Why?
>
> -- Yoshiki
>
>