Pass a format do GdkTextureClass::download(). That way we can download
data in any format.
Also replace gdk_texture_download_texture() with
gdk_memory_texture_from_texture() which again takes a format.
The old functionality is still there for code that wants it: Just pass
gdk_texture_get_format (texture) as the format argument.
For MemoryTexture, this is a simple change.
For GLTexture, we need to query the format at texture creation. This
sounds like a bad idea and extra work until one realizes that we'd
need to do that anyway when using the texure the first time - either
when downloading, or when trying to use it in a rendernode, where we
will soon need that information to determine if the texture prefers high
depth.
A private vfunc that downloads a texture as a GdkMemoryTexture in
whatever format the texture deems best.
There are multiple reasons for this:
* GLES cannot download the Cairo format. But it can download some
format and then just delegate to the GdkMemoryTexture implementation.
* All the other download vfuncs (including the ones still coming) can
be implemented via download_texture() and delegation, making the
interface easier.
* We want to implement image loading and saving support. By using
download_texture(), we can save in the actual format of the texture.
* A potential GdkCompressedTexture could be implemented by just
providing this one vfunc as a compress() step.
A problem with textures is that they can become too big for GPU memory,
which would require tiling. But for tiling we only need to download
the pixels needed by the tile.
Similarly, there might be interest to not upload full textures if a
renderer knows it only needs a small part.
Both of these methods require the ability to specify an area of the
texture to be downloaded. So change the download vfunc to include
this parameter now before we add even more textures later.
A private gdk_texture_download_area() function has also been added, but
nobody is using it yet.
Put GdkGLTexture into its own file and rename the API to
gdk_gl_texture_foo() instead of gdk_texture_foo_for_gl().
Apart from naming, no actual code changes.
