Downloads the this into local memory.
This may be an expensive operation, as the actual texture data may reside on a GPU or on a remote display server.
The data format of the downloaded data is equivalent to cairo_format_argb32, so every downloaded pixel requires 4 bytes of memory.
Downloading a texture into a Cairo image surface: ```c surface = cairo_image_surface_create (CAIRO_FORMAT_ARGB32, gdk_texture_get_width ( texture), gdk_texture_get_height (texture)); gdk_texture_download (texture, cairo_image_surface_get_data (surface), cairo_image_surface_get_stride (surface)); cairo_surface_mark_dirty (surface); ```
pointer to enough memory to be filled with the downloaded data of this
rowstride in bytes