Set wxWebResponseCURL buffer size as soon as known

Currently for wxWebRequestCURL objects using memory storage, a download
is processed by appending to a wxMemoryBuffer each time the write
callback is called. For a large transfer, this can result in many, many
reallocation calls and can block the main application.

This commit uses the progress callback for wxWebRequestCURL objects
added in a previous commit to set a minimum size for the buffer as soon
as it is known.

include/wx/private/webrequest.h

@@ -187,6 +187,10 @@ protected:
 
     void ReportDataReceived(size_t sizeReceived);
 
+    // This function can optionally be called to preallocate the read buffer,
+    // if the total amount of data to be downloaded is known in advance.
+    void PreAllocBuffer(size_t sizeNeeded);
+
 private:
     // Called by wxWebRequestImpl only.
     friend class wxWebRequestImpl;