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

cogl/cogl-glsl-shader-private.h

@@ -25,7 +25,6 @@
 
 void
 _cogl_glsl_shader_set_source_with_boilerplate (CoglContext *ctx,
-                                               const char *version_string,
                                                GLuint shader_gl_handle,
                                                GLenum shader_gl_type,
                                                CoglPipeline *pipeline,