chore: simplify alpha premultiplication, now supported by sharp (#1073)

Maplibre-native outputs premultiplied pixels values. The sharp library did not
support it so we added code to cancel the alpha premultiplication.
Note that this can only visible onr raster tiles (and probably static maps).

The sharp library now supports premultiplied pixels with the right config.
Let's use it: it should be faster and easie to maintain.

Feature announced here:
lovell/sharp#1599 (comment)

Feature developped here by @mnutt:
lovell/sharp#2685

Signed-off-by: Martin d'Allens <martin.dallens@liberty-rider.com>

src/serve_rendered.js

@@ -428,24 +428,9 @@ const respondImage = (
         return res.status(500).header('Content-Type', 'text/plain').send(err);
       }
 
-      // Fix semi-transparent outlines on raw, premultiplied input
-      // https://github.com/maptiler/tileserver-gl/issues/350#issuecomment-477857040
-      for (let i = 0; i < data.length; i += 4) {
-        const alpha = data[i + 3];
-        const norm = alpha / 255;
-        if (alpha === 0) {
-          data[i] = 0;
-          data[i + 1] = 0;
-          data[i + 2] = 0;
-        } else {
-          data[i] = data[i] / norm;
-          data[i + 1] = data[i + 1] / norm;
-          data[i + 2] = data[i + 2] / norm;
-        }
-      }
-
       const image = sharp(data, {
         raw: {
+          premultiplied: true,
           width: params.width * scale,
           height: params.height * scale,
           channels: 4,