Crashfix when importing/selecting large png

Uses size_t as per Thomas Holder's suggestion.
Fixes gitlab.com/inkscape/inkscape/issues/192.

(backported from 90705c91)

src/display/cairo-utils.cpp

@@ -1240,7 +1240,7 @@ guint32 pixbuf_from_argb32(guint32 c)
 void
 convert_pixels_pixbuf_to_argb32(guchar *data, int w, int h, int stride)
 {
-    for (int i = 0; i < h; ++i) {
+    for (size_t i = 0; i < h; ++i) {
         guint32 *px = reinterpret_cast<guint32*>(data + i*stride);
         for (int j = 0; j < w; ++j) {
             *px = argb32_from_pixbuf(*px);
@@ -1257,7 +1257,7 @@ convert_pixels_pixbuf_to_argb32(guchar *data, int w, int h, int stride)
 void
 convert_pixels_argb32_to_pixbuf(guchar *data, int w, int h, int stride)
 {
-    for (int i = 0; i < h; ++i) {
+    for (size_t i = 0; i < h; ++i) {
         guint32 *px = reinterpret_cast<guint32*>(data + i*stride);
         for (int j = 0; j < w; ++j) {
             *px = pixbuf_from_argb32(*px);

src/display/drawing-image.cpp

@@ -208,7 +208,7 @@ DrawingImage::_pickItem(Geom::Point const &p, double delta, unsigned /*sticky*/)
         unsigned char *const pixels = _pixbuf->pixels();
         int width = _pixbuf->width();
         int height = _pixbuf->height();
-        int rowstride = _pixbuf->rowstride();
+        size_t rowstride = _pixbuf->rowstride();
 
         Geom::Point tp = p * _ctm.inverse();
         Geom::Rect r = bounds();