[cogl-vertex-buffer] fix n_components validation for GL's builtin attributes

[cogl-vertex-buffer] fix n_components validation for GL's builtin attributes

glVertexPointer expects positions with 2, 3 or 4 components, glColorPointer
expects colors with 3 or 4 components and glNormalPointer expects normals
with three components so when adding vertex buffer atributes with the names
"gl_Vertex", "gl_Color" or "gl_Normal" we assert these constraints and print
an explanation to the developer if not met.

This also fixes the previosly incorrect constraint that gl_Normal attributes
must have n_components == 1; thanks to Cat Sidhe for reporting this:

Bug: http://bugzilla.openedhand.com/show_bug.cgi?id=1819