introduce texture loaders to make allocations lazy

This introduces the internal idea of texture loaders that track the
state for loading and allocating a texture. This defers a lot more work
until the texture is allocated.

There are several intentions to this change:

- provides a means for extending how textures are allocated without
  requiring all the parameters to be supplied in a single _texture_new()
  function call.

- allow us to remove the internal_format argument from all
  _texture_new() apis since using CoglPixelFormat is bad way of
  expressing the internal format constraints because it is too specific.

  For now the internal_format arguments haven't actually been removed
  but this patch does introduce replacement apis for controlling the
  internal format:

    cogl_texture_set_components() lets you specify what components your
    texture needs when it is allocated.
    cogl_texture_set_premultiplied() lets you specify whether a texture
    data should be interpreted as premultiplied or not.

- Enable us to support asynchronous texture loading + allocation in the
  future.

Of note, the _new_from_data() texture constructors all continue to
allocate textures immediately so that existing code doesn't need to be
adapted to manage the lifetime of the data being uploaded.

Reviewed-by: Neil Roberts <neil@linux.intel.com>
(cherry picked from commit 6a83de9ef4210f380a31f410797447b365a8d02c)

Note: Compared to the original patch, the ->premultipled state for
textures isn't forced to be %TRUE in _cogl_texture_init since that
effectively ignores the users explicitly given internal_format which was
a mistake and on master that change should have been made in the patch
that followed. The gtk-doc comments for cogl_texture_set_premultiplied()
and cogl_texture_set_components() have also been updated in-line with
this fix.

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