Minor changes in RSXTexture.h and PKG Loader

* Moved RSXTexture from RSXThread.h to RSXTexture.h
* Renamed RSXTexture::Getmipmap to RSXTexture::GetMipmap
* Used more GCM texture format constants in GLGSRender.h
* PKG Loader: Added an "Overwrite?" dialog in case the folder
dest+titleID (i.e. "/dev_hdd0/game/"+titleID) already exists. Note that
in that case, the game won't be installed regardless of the option you
choose on the dialog.