cogl-vertex-buffer: Refix disabling texture coord arrays

cogl-vertex-buffer: Refix disabling texture coord arrays

Commit 92a375ab4 changed the initial value of max_texcoord_attrib_unit
to -1 so that it could disable the texture coord array for the first
texture unit when there are no texture coords used in the vbo. However
max_texcoord_attrib_unit was an unsigned value so this actually became
G_MAXUINT. The disabling loop at the bottom still worked because
G_MAXUINT+1==0 but the check for whether any texture unit is greater
than max_texcoord_attrib_unit was failing so it would always end up
disabling all texture units. This is now fixed by changing
max_texcoord_attrib_unit to be signed.