Add a texture coordinate generation mode to always use
4-component homogeneous texture coordinates on OpenGL.

Bug 21187909

Change-Id: I58714404002102f4138add454cf5a8c0aecfb062
Reviewed-on: https://swiftshader-review.googlesource.com/3145Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Maxime Grégoire <mgregoire@google.com>
Tested-by: Maxime Grégoire <mgregoire@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Greg Hartman <ghartman@google.com>
Reviewed-by: Greg Hartman <ghartman@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>