gl: Fix one off issue in context cleanup

ctx->vertex_shaders is only CAIRO_GL_VAR_TYPE_MAX large, so we should
abort the loop before the index is equal to CAIRO_GL_VAR_TYPE_MAX.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Correct usage of CAIRO_STACK_ARRAY_LENGTH

This macro wants the array type as its argument and calls sizeof() on it
internally.

Signed-off-by: Uli Schlachter <psychon@znc.in>