docs: Fix the documentation for texture coordinates in snippets

The documentation for the builtin varyings for the texture coordinates
was wrongly claiming that the varyings are stored in an array. This
was changed in e55b64a9cdc9 so that each layer gets its own
independent varying.

The documentation was also referring to texture units instead of layer
numbers. The texture units are no longer publicly exposed in the
shaders and instead everything should in theory be expressed in terms
of layer numbers.

Reviewed-by: Robert Bragg <robert@linux.intel.com>
(cherry picked from commit bf6b509c340bdc3be30e1a81fb96710b3176e9dc)
Always add the #version pragma to shaders

Previously we would only add the #version pragma to shaders when
point sprite texture coordinates are enabled for a layer so that we
can access the gl_PointCoord builtin. However I don't think there's
any good reason not to just always request GLSL version 1.2 if it's
available. That way applications can always use gl_PointCoord without
having to enable point sprite texture coordinates.

This adds a glsl_version_to_use member to CoglContext which is used to
generate the #version pragma as part of the shader boilerplate. On
desktop GL this is set to 120 if version 1.2 is available, otherwise
it is left at 110. On GLES it is always left as 100.

Reviewed-by: Robert Bragg <robert@linux.intel.com>
(cherry picked from commit e4dfe8b07e8af111ecbcb0da20ff2a2875a2b5d0)

Conflicts:
	cogl/driver/gl/gl/cogl-driver-gl.c
Add the cogl_point_coord snippet builtin

This adds a #define for gl_PointCoord to all shaders so that it can be
accessed with a name in the Cogl namespace.

Reviewed-by: Robert Bragg <robert@linux.intel.com>
(cherry picked from commit c28fc054788e88627bcc2346f4c4c368870ff777)
Add a conformance test for using cogl_point_coord in a shader

This adds a modification to the test-point-sprite test which uses a
shader snippet which directly references cogl_point_coord instead of
relying on cogl_pipeline_set_layer_point_sprite_coords_enabled to
replace the texture coordinates with the point coords.

Reviewed-by: Robert Bragg <robert@linux.intel.com>
(cherry picked from commit 5ea57b14f5ec7d52ae378f4ca64574ef56342f56)
Add Autotools Items to Complete the Cogl-Path MSVC Projects

This adds a centralized autotools Makefile to be used in the completion
of the project files for Cogl-Path for now, which can be used also in
Cogl, Cogl-Pango and possibly Cogl-Gst so that we could have less clutter
in the autotools files.

This patch set will also allow the Cogl-Path project files to be filled in
during 'make dist' and make them distributed during a tarball release.

Reviewed-by: Neil Roberts <neil@linux.intel.com>
(cherry picked from commit 491020592b240ca5f2885dee104c88bf431858ae)

Conflicts:
	cogl-path/Makefile.am