Fixed Mesh move constructor/assignment.

Missed copying of base vertex.
Fix usage of reserved GLSL keyword in test.
Be less strict in OpenGL ES context version.

I didn't yet have time to add more complete support for ES 3.1, but the
engine should at least work on it. If the driver reports ES 3.0 or 3.1
it is taken as valid value in both ES2 and ES3 builds (i.e. the engine
can work in ES2 mode also on ES3 driver).
Doc++
Cleanup and code reorganization in MeshGLTest.
glDrawElementsBaseVertex() needs ARB_draw_elements_base_vertex.
Support for instanced mesh drawing and base instance.

On desktop GL this is provided by ARB_draw_instanced (GL 3.1). Base
instance is available only on desktop GL (4.2, ARB_base_instance). In
ES2 the instanced functionality is provided by three (!) different
extensions (ANGLE_instanced_arrays, EXT_draw_instanced,
NV_draw_instanced), the proper implementation is chosen on context
creation based on what extension is available. Though we don't have
extension loader for ES yet, thus all these extensions are disabled and
the implementation has assertion in it.

Added blind test which tests only that something has been drawn and no
errors were emitted, but not whether the right command is used. I'll
probably need to check this later, because the Mesh::draw() behemoth is
going slightly out of hand :)
Code reorganization.
Added Mesh::addVertexBufferInstanced().

Functionality provided by GL 3.3 and ARB_instanced_arrays, on ES2 this
is again implemented in three different extensions --
{ANGLE,EXT,NV}_instanced_arrays. They are disabled until Magnum has
proper extension loading on ES.