evas_canvas3d: Unification algorithm generation shadow

evas_canvas3d: Unification algorithm generation shadow

Summary:
Change type of texdepth from GL_R(doesn't support with GLES) to GL_RGBA.
Don't use useless additional framebuffers for shadows.
Use same shader code for generation shadow map texture.
Turn always using software alpha-test.
It reduce and simplify code in general.

Reviewers: cedric, Hermet

Subscribers: cedric

Differential Revision: https://phab.enlightenment.org/D3115