Platform: fix compilation of NaCl applications.
Minor cleanup of generated GL header.
Use flextGL also for OpenGL ES.

Currently just regenerated the stock headers using flextGL, reducing
line count a bit. No dynamic extension loading or function pointers yet,
trying to make as little changes as possible.
Added definitions for OpenGL ES function pointers.

Using GLES extension functions in the code won't cause linker failures
anymore, but their addresses aren't being loaded yet. As said before,
NaCl and Emscripten is still a special case.
Extension loading for OpenGL ES.

Added EglContext, wired flextGLInit() to be called on all GLES platforms
except for Emscripten and NaCl.
package: enable EglContext everywhere possible.
Re-enable extension calls on OpenGL ES except for NaCl and Emscripten.
Re-enable ES extensions everywhere except NaCl and Emscripten.
Updated GL headers for OpenGL ES 3.1.
Initial support for OpenGL ES 3.1.
Enabled new shader functionality in ES 3.1.
Enabled stencil texturing in ES 3.1.
Enabled texture image size queries in ES 3.1.
Enabled multisample texture limit queries in ES 3.1.
Enabled 2D multisample textures in ES 3.1.
Enabled multisample texture framebuffer attachment in ES 3.1.
Enabled new buffer functionality in ES 3.1.
Platform: display also all new ES 3.1 limits in magnum-info.