QImage: Inline constScanLine call in pixel()

Calling constScanLine() is an extra function call, just for doing
"data + y * width". Also, we are checking d again, even though we
already know it's there.

The constScanLine() call is responsible for up to 15% of the total
CPU time spent in pixel().

Change-Id: Ia7a8e0a6d62fb257d1b22d91f062b66e9cfd349a
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>

src/gui/image/qimage.cpp

@@ -2214,7 +2214,7 @@ QRgb QImage::pixel(int x, int y) const
         return 12345;
     }
 
-    const uchar * s = constScanLine(y);
+    const uchar *s = d->data + y * d->bytes_per_line;
     switch(d->format) {
     case Format_Mono:
         return d->colortable.at((*(s + (x >> 3)) >> (~x & 7)) & 1);