Add support for bit depth with raw input and output (#2762)

* Determine input raw pixel depth from the given typed array
* Allow pixel depth to be set on raw output

src/common.h

@@ -54,6 +54,7 @@ namespace sharp {
     size_t bufferLength;
     bool isBuffer;
     double density;
+    VipsBandFormat rawDepth;
     int rawChannels;
     int rawWidth;
     int rawHeight;
@@ -78,6 +79,7 @@ namespace sharp {
       bufferLength(0),
       isBuffer(FALSE),
       density(72.0),
+      rawDepth(VIPS_FORMAT_UCHAR),
       rawChannels(0),
       rawWidth(0),
       rawHeight(0),