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