cogl-shader-boilerplate: Specify default precision earlier

In 9ff04e8a the builtin uniforms were moved to the common shader
boilerplate. However the common boilerplate is positioned before the
default precision specifier on GLES2 so it would fail to compile
because the uniforms end up with no precision in the fragment
shader. This patch just moves the precision specifier to above the
common boilerplate.

(cherry picked from commit d1500c03)