OpenGL/Canvas: use GL_SHORT for VBO

Fixes regression by previous commit.
OpenGL/Program: add attribute methods
OpenGL/Shaders: library managing global shaders

More shaders to come.
OpenGL/Canvas: GLES2 implementation of ScopeVertexPointer
OpenGL/Canvas: disable glTexEnvi() and GL_LOGIC_OP on GLES 2.0

Replaced with TODO comments, to be implemented later.