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_curl.h

@@ -114,11 +114,12 @@ public:
     // Methods called from libcurl callbacks
     size_t CURLOnWrite(void *buffer, size_t size);
     size_t CURLOnHeader(const char* buffer, size_t size);
-    int CURLOnProgress();
+    int CURLOnProgress(curl_off_t);
 
 private:
     wxWebRequestHeaderMap m_headers;
     wxString m_statusText;
+    wxFileOffset m_knownDownloadSize;
 
     CURL* GetHandle() const
     { return static_cast<wxWebRequestCURL&>(m_request).GetHandle(); }