mesa: fix GLSL issue preventing use of all 16 generic vertex attributes

mesa: fix GLSL issue preventing use of all 16 generic vertex attributes

Only 15 actually worked before since we always reserved generic[0] as an
alias for vertex position.

The case of vertex attribute 0 is tricky.  The spec says that there is no
aliasing between generic vertex attributes 0..MAX_VERTEX_ATTRIBS-1 and the
conventional attributes.  But it also says that calls to glVertexAttrib(0, v)
are equivalent to glVertex(v).  The distinction seems to be in glVertex-mode
versus vertex array mode.

So update the VBO code so that if the shader uses generic[0] but not gl_Vertex,
route the attribute data set with glVertex() to go to shader input generic[0].

No change needed for the glDrawArrays/Elements() path.

This is a potentially risky change so regressions are possible.  All the usual
tests seem OK though.