document that CanRead() and GetImageCount() functions of wxImageHandlers do NOT modify the current stream position and that they require seekable streams; rename current GetImageCount() functions to DoGetImageCount() and put save-and-restore stream position logic in GetImageCount(); add comments in the various DoCanRead() and in DoGetImageCount() where the stream position is modified; remove unneeded SeekI(0) calls from DoCanRead() and DoGetImageCount() functions: they didn't allow to load images from non-seekable streams; implement forward-seeking in wxInputStream::SeekI() also for non-seekable streams

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@60852 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/stream.h

@@ -76,7 +76,7 @@ public:
     virtual bool IsOk() const;
 
     /**
-        Returns @true if the streams supports seeking to arbitrary offsets.
+        Returns @true if the stream supports seeking to arbitrary offsets.
     */
     virtual bool IsSeekable() const;
 
@@ -613,6 +613,11 @@ public:
     /**
         Changes the stream current position.
 
+        This operation in general is possible only for seekable streams 
+        (see wxStreamBase::IsSeekable()); non-seekable streams support only
+        seeking positive amounts in mode @c wxFromCurrent (this is implemented
+        by reading data and simply discarding it).
+
         @param pos
             Offset to seek to.
         @param mode
@@ -623,7 +628,9 @@ public:
     virtual wxFileOffset SeekI(wxFileOffset pos, wxSeekMode mode = wxFromStart);
 
     /**
-        Returns the current stream position.
+        Returns the current stream position or ::wxInvalidOffset if it's not
+        available (e.g. socket streams do not have a size nor a current stream
+        position).
     */
     virtual wxFileOffset TellI() const;