cogl-texture-2d-sliced: Use the other backends for the slices

Instead of directly manipulating GL textures itself,
CoglTexture2DSliced now works in terms of CoglHandles. It creates the
texture slices using cogl_texture_new_with_size which should always
end up creating a CoglTexture2D because the size should fit. This
allows us to avoid replicating some code such as the first pixel
mipmap tracking and it better enforces the separation that each
texture backend is the only place that contains code dealing with each
texture target.

cogl/cogl-texture-2d-sliced-private.h

@@ -38,29 +38,11 @@ struct _CoglTexture2DSliced
   CoglTexture       _parent;
   GArray           *slice_x_spans;
   GArray           *slice_y_spans;
-  GArray           *slice_gl_handles;
+  GArray           *slice_textures;
   int               max_waste;
 
-  /* The internal format of the GL texture represented as a
-     CoglPixelFormat */
-  CoglPixelFormat   format;
-  /* The internal format of the GL texture represented as a GL enum */
-  GLenum            gl_format;
-  GLenum            gl_target;
   int               width;
   int               height;
-  GLenum            min_filter;
-  GLenum            mag_filter;
-  gboolean          is_foreign;
-  GLenum            wrap_mode_s;
-  GLenum            wrap_mode_t;
-  gboolean          auto_mipmap;
-  gboolean          mipmaps_dirty;
-
-  /* This holds a copy of the first pixel in each slice. It is only
-     used to force an automatic update of the mipmaps when
-     glGenerateMipmap is not available. */
-  CoglTexturePixel *first_pixels;
 };
 
 GQuark