glfiltercube: the cube does not look like a cube

Submitted by Julien Isorce @cap

Link to original bug (#752745)

Description

glfiltercube does not output a cube :)

Instead of properly computing the Model-View-Projection matrixes, we currently provide this fixed matrix: http://cgit.freedesktop.org/gstreamer/gst-plugins-bad/tree/ext/gl/gstglfiltercube.c#n454

The cube was showing ok when not using shaders but since we moved to use shaders not only on gles2 I realize the cube was already wrong with gles2+GstGL-0.10 (see http://cgit.freedesktop.org/gstreamer/attic/gst-plugins-gl/commit/gst/gl/gstglfiltercube.c?id=e12532c6c5bb0fcbbf94abb6aa8e98757e536d1d)

But it requires to manually do some maths. See http://cgit.freedesktop.org/gstreamer/gst-omx/tree/examples/egl/testegl.c#n123
Maybe we can add gst_gl_matrix_load_identity/gst_gl_matrix_multiply/gst_gl_matrix_translate/gst_gl_matrix_frustum/gst_gl_matrix_perspective

It seems to me to be "a small piece of library API there" (from "master feature freeze email") but not sure.
Otherwise we keep these functions locally in glfiltercube and move them to GstGL after 1.6, see following patch.