[XAM] Write stub buffer in XamReadTileToTexture.

[XAM] Write stub texture buffer (white) in XamReadTileToTexture.

src/xenia/kernel/xam/xam_user.cc

@@ -737,6 +737,10 @@ dword_result_t XamReadTileToTexture(dword_t unknown, dword_t title_id,
                                     lpvoid_t buffer_ptr, dword_t stride,
                                     dword_t height, dword_t overlapped_ptr) {
   // TODO(gibbed): unknown=0,2,3,9
+
+  size_t size = size_t(stride) * size_t(height);
+  std::memset(buffer_ptr, 0xFF, size);
+
   if (overlapped_ptr) {
     kernel_state()->CompleteOverlappedImmediate(overlapped_ptr,
                                                 X_ERROR_SUCCESS);