cogl-shader: Prepend boilerplate for portable shaders

We now prepend a set of defines to any given GLSL shader so that we can
define builtin uniforms/attributes within the "cogl" namespace that we
can use to provide compatibility across a range of the earlier versions
of GLSL.

This updates test-cogl-shader-glsl.c and test-shader.c so they no longer
needs to special case GLES vs GL when splicing together its shaders as
well as the blur, colorize and desaturate effects.

To get a feel for the new, portable uniform/attribute names here are the
defines for OpenGL vertex shaders:

 #define cogl_position_in gl_Vertex
 #define cogl_color_in gl_Color
 #define cogl_tex_coord_in  gl_MultiTexCoord0
 #define cogl_tex_coord0_in gl_MultiTexCoord0
 #define cogl_tex_coord1_in gl_MultiTexCoord1
 #define cogl_tex_coord2_in gl_MultiTexCoord2
 #define cogl_tex_coord3_in gl_MultiTexCoord3
 #define cogl_tex_coord4_in gl_MultiTexCoord4
 #define cogl_tex_coord5_in gl_MultiTexCoord5
 #define cogl_tex_coord6_in gl_MultiTexCoord6
 #define cogl_tex_coord7_in gl_MultiTexCoord7
 #define cogl_normal_in gl_Normal

 #define cogl_position_out gl_Position
 #define cogl_point_size_out gl_PointSize
 #define cogl_color_out gl_FrontColor
 #define cogl_tex_coord_out gl_TexCoord

 #define cogl_modelview_matrix gl_ModelViewMatrix
 #define cogl_modelview_projection_matrix gl_ModelViewProjectionMatrix
 #define cogl_projection_matrix gl_ProjectionMatrix
 #define cogl_texture_matrix gl_TextureMatrix

And for fragment shaders we have:

 #define cogl_color_in gl_Color
 #define cogl_tex_coord_in gl_TexCoord

 #define cogl_color_out gl_FragColor
 #define cogl_depth_out gl_FragDepth

 #define cogl_front_facing gl_FrontFacing

cogl/cogl-pipeline-opengl.c

@@ -1036,7 +1036,8 @@ backend_add_layer_cb (CoglPipelineLayer *layer,
  */
 void
 _cogl_pipeline_flush_gl_state (CoglPipeline *pipeline,
-                               gboolean skip_gl_color)
+                               gboolean skip_gl_color,
+                               int n_tex_coord_attribs)
 {
   unsigned long    pipelines_difference;
   int              n_layers;
@@ -1139,7 +1140,8 @@ _cogl_pipeline_flush_gl_state (CoglPipeline *pipeline,
        * scratch buffers here... */
       if (G_UNLIKELY (!backend->start (pipeline,
                                        n_layers,
-                                       pipelines_difference)))
+                                       pipelines_difference,
+                                       n_tex_coord_attribs)))
         continue;
 
       state.backend = backend;