texture-rectangle: update _new_with_size in line with master

This updates the cogl_texture_rectangle_new_with_size() api in line with
master to be consistent with other texture constructors. This removes
the internal_format and error arguments and allows the texture to be
allocated lazily which means the texture can be configured with apis
like cogl_texture_set_components() and cogl_texture_set_premultiplied()
before it is allocated.

Reviewed-by: Neil Roberts <neil@linux.intel.com>
texture: fix cogl_texture_get_components prototype

This fixes the cogl_texture_get_components() prototype to have a return
type of CoglTextureComponents instead of CoglBool which was probably a
copy and paste error.

(cherry picked from commit 55b09f8a939db71ee5ff41afa0ed08cbe937a4ec)
pipeline-cache: Prune old unused pipelines when the cache gets too big

Previously when a pipeline is added to the cache it would never be
removed. If the application is generating a lot of unique pipelines
this can end up effectively leaking a large number of resources
including the GL program objects. Arguably this isn't really a problem
because if the application is generating that many unique pipelines
then it is doing something wrong anyway. It also implies that it will
be recompiling shaders very often so the cache leaking will likely be
the least of the problems.

This patch makes it keep track of which pipelines in the cache are in
use. The cache now returns a struct representing the entry instead of
directly returning the pipeline. This entry contains a usage counter
which the pipeline backends can use to mark when there is a pipeline
alive that is using the cache entry. When the hash table decides that
it's a good time to prune some entries, it will make a list of all of
the pipelines that are not in use and then remove the least recently
used half of the pipelines. That way it is less likely to remove
pipelines that the application is actually regenerating often even if
they aren't in use all of the time.

When the cache is pruned the hash table makes a note of how small the
cache could be if it removed all of the unused pipelines. The hash
table starts pruning when there are more entries than twice this
minimum expected size. The idea is that if that case it hit then the
hash table is more than half full of useless pipelines so the
application is generating lots of redundant pipelines and it is a good
time to remove them.

Reviewed-by: Robert Bragg <robert@linux.intel.com>

(cherry picked from commit c21aac22992bb7fef5a8d0913130b8245e67f2eb)

Conflicts:
	cogl/driver/gl/cogl-pipeline-fragend-glsl.c
	cogl/driver/gl/cogl-pipeline-progend-glsl.c
	cogl/driver/gl/cogl-pipeline-vertend-glsl.c
	cogl/driver/gl/gl/cogl-pipeline-fragend-arbfp.c
Tweak documentation for the CoglTexture interface

The following changes are made to the documentation for CoglTexture:

• The description of the default value for the components property is
  changed to say that it is always RGBA for textures created by the
  ‘_with_size’ textures. Previously it said that the default is based
  on the pixel format used the first time data is set on the texture,
  but this is only true if the data is set using a constructor.

• Added documentation for the CoglTextureComponents enum.

• Changed it to say that it _specifies_ what components are required
  for sampling rather than determinging [sic] them.

• Added ‘Since: 1.18’ to
  cogl_texture_{set,get}_{components,premultiplied}

• Changed the since tag for CoglTextureError from 2.0 to 1.8.

• Added documentation for COGL_TEXTURE_ERROR_{FORMAT,TYPE}.

• Added the following to the cogl2-sections.txt file:
  COGL_TEXTURE_ERROR
  CoglTextureError
  cogl_texture_allocate
  cogl_texture_set_components
  cogl_texture_get_components
  cogl_texture_set_premultiplied
  cogl_texture_get_premultiplied

Reviewed-by: Robert Bragg <robert@linux.intel.com>
(cherry picked from commit 2f12c4329c519fa14b927b2dcd708dddcc903c32)
Add support for RG textures

This adds COGL_PIXEL_FORMAT_RG_88 and COGL_TEXTURE_COMPONENTS_RG in
order to support two-component textures. The RG components for a
texture is only supported if COGL_FEATURE_ID_TEXTURE_RG is advertised.
This is only available on GL 3, GL 2 with the GL_ARB_texture_rg
extension or GLES with the GL_EXT_texture_rg extension. The RG pixel
format is always supported for images because Cogl can easily do the
conversion if an application uses this format to upload to a texture
with a different format.

If an application tries to create an RG texture when the feature isn't
supported then it will raise an error when the texture is allocated.

https://bugzilla.gnome.org/show_bug.cgi?id=712830

Reviewed-by: Robert Bragg <robert@linux.intel.com>

(cherry picked from commit 568677ab3bcb62ababad1623be0d6b9b117d0a26)

Conflicts:
	cogl/cogl-bitmap-packing.h
	cogl/cogl-types.h
	cogl/driver/gl/gl/cogl-driver-gl.c
	tests/conform/test-read-texture-formats.c
	tests/conform/test-write-texture-formats.c