More sensible defaults and allow for overriding. Also make PDB installation
optional on Windows if RelWithDebInfo isn't the build type being used.
CMAKE_INSTALL_PREFIX defaults to GStreamer install location on Windows and
/usr/lib on other platforms.
PLUGIN_INSTALL_DIR defaults to CMAKE_INSTALL_PREFIX/gstreamer-1.0
LIBRARY_INSTALL_DIR defaults to CMAKE_INSTALL_PREFIX/
PDB_INSTALL_DIR defaults to CMAKE_INSTALL_PREFIX/
Since we're using the Imperx unpack function, we can easily handle
packed pixel formats. For performance reasons, we could always support
packed pixel formats directly by adding new caps.
Some newer version of GStreamer requires the plugin name used in
GST_PLUGIN_DEFINE to match the name of the compiled module, excluding
libgst and the extension.
