Fix point sprite coordinates in shaders on GLES2

When using the GLES2 driver with a pipeline layer that has point
sprite coordinates enabled it extracts the texture coordinates from
the gl_PointCoord builtin instead of cogl_tex_coord_in[*].
gl_PointCoord is not quite the same as the regular texture coordinates
because it is only a vec2 instead of a vec4. This used to work ok
because either set of texture coordinates would always immediately be
accessed using a swizzle of '.st' so it would effectively be converted
to a vec2 anyway. However since the snippet hook for texture lookups
was added the texture coordinates are now passed to a function to
perform the actual lookup instead. This function always takes a vec4
so the shader would not compile. This patch fixes it to construct a
vec4 from gl_PointCoord by setting the third and fourth components to
0.0 and 1.0.

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

(cherry picked from commit cb3409b23caf324a548f82cfb88acc684cc71930)

cogl/cogl-pipeline-fragend-glsl.c

@@ -455,7 +455,7 @@ ensure_texture_lookup_generated (CoglPipelineShaderState *shader_state,
       cogl_pipeline_get_layer_point_sprite_coords_enabled (pipeline,
                                                            layer->index))
     g_string_append_printf (shader_state->source,
-                            "gl_PointCoord");
+                            "vec4 (gl_PointCoord, 0.0, 1.0)");
   else
     g_string_append_printf (shader_state->source,
                             "cogl_tex_coord_in[%d]",