framebuffer: if size unknown; allocate for size/vp queries

This ensures framebuffers are implicitly allocated when querying the
width, height or viewport width/height if the framebuffer's size is
currently unknown. The plan is to allow texture backends to defer
calculating the size of textures until they are allocated which in turn
means we won't know the size of offscreen framebuffers until the texture
has been allocated. Potentially we could be more specific about this in
the future and only ensure the texture is allocated, but for now it will
be simplest to just ensure the framebuffer is allocated.

Note: in the case of onscreen buffers which are always initialized with
a requested size we are careful to avoid triggering an allocation when
this is queried otherwise we will see recursion when the winsys code
queries the requested size during allocation.
framebuffer: defer calculating level size until allocation

The plan is to defer more of the work for creating a texture until
allocation time, but that means we won't be able to always assume
we can query the size of a texture when creating an offscreen
framebuffer from a texture (consider for example using
_texture_new_from_file() where the size isn't known until the file has
been loaded). This defers needing to know the size of the texture
underlying an offscreen framebuffer until calling
cogl_framebuffer_allocate().
ensure texture allocated in _get_gl_texture() in preparation

The plan is to defer a lot more work in creating a texture until
allocation time. This means we wont be able to assume that all textures
being used to render must have already been allocated when data was
specified.

The latest point at which we will generally require a texture to be
allocated will be when we need to know the underlying GL handle for a
texture and so this updates cogl_texture_get_gl_texture() to ensure the
texture is allocated.
texture: allocate on slicing/hw repeat queries

The plan is to defer a lot more work in creating a texture until
allocation time. This means that for some texture backends we might not
know until after allocation whether the texture is sliced or can support
hardware repeating. This makes sure we trigger an allocation if either
of these are queried.
atlas: make zero size a programmer error

Instead of throwing a CoglError exception if an application tries to
allocate a zero size atlas texture this make that a programmer error
instead.
atlas: defer checks until allocation

This defers checking the internal format and whether accelerated
migration is supported until allocating the texture.
texture: make cogl_texture_get_format api private

CoglPixelFormat is not a good way of describing the internal
format of a texture because it's too specific given that we don't
actually have exact knowledge of the internal format used by the driver.

This makes cogl_texture_get_format private and in the future we'll
provide a better way of querying the channels and their precision.
texture: return ints from _get_width/height

This changes cogl_texture_get_width/height to return signed integers
just because unsigned integers in C often cause un-obvious arithmetic
issues due to implicit casting.
get_texture_bits_via_offscreen(): use meta texture format

When reading a texture back by first wrapping it as an offscreen
framebuffer and using _read_pixels_into_bitmap() we now make sure the
offscreen framebuffer has an internal format that matches the
meta-texture being read not that of the current sub-texture being
iterated. In the case of atlas textures the subtexture is a shared
texture whose format doesn't reflect the premultipled alpha status of
individual atlas-textures, nor whether the alpha component is valid.
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.
remove internal_format and redundant error arguments

Texture allocation is now consistently handled lazily such that the
internal format can now be controlled using
cogl_texture_set_components() and cogl_texture_set_premultiplied()
before allocating the texture with cogl_texture_allocate(). This means
that the internal_format arguments to texture constructors are now
redundant and since most of the texture constructors now can't ever fail
the error arguments are also redundant. This now means we no longer
use CoglPixelFormat in the public api for describing the internal format
of textures which had been bad solution originally due to how specific
CoglPixelFormat is which is missleading when we don't support such
explicit control over the internal format.