JoglGL20 JoglGraphicsBase: try GLES2 or GL2 context first before picking a OpenGL core only GL2ES2 context

Fixes shader loading for Animation3Dtest, on backward compatible OpenGL systems.
Fixes exception not a GL2 context while running TreeTest.
Make code GLES2 compatible.

libgdx uses glDrawElements and glVertexAttribPointer passing buffers
these functions are removed in OpenGL core only contexts
libgdx shaders are currently only GLES2 and GL2 compatible.
try allocate an GLES2 or GL2 context first
before picking a OpenGL core only GL2ES2 context.

Signed-off-by: Xerxes Rånby <xerxes@zafena.se>