Increase default buffer size for wxWebRequest operations

Use 64KiB rather than 8KiB, as the latter seems rather small nowadays.

Also add a symbolic constant for this number.

include/wx/private/webrequest.h

@@ -18,6 +18,9 @@
 
 WX_DECLARE_STRING_HASH_MAP(wxString, wxWebRequestHeaderMap);
 
+// Default buffer size when a fixed-size buffer must be used.
+const int wxWEBREQUEST_BUFFER_SIZE = 64 * 1024;
+
 // ----------------------------------------------------------------------------
 // wxWebAuthChallengeImpl
 // ----------------------------------------------------------------------------