add a [context name( message to switch rendering context

since Pd's [receive] doesn't have a [set( method (which is a pity)
add this to switch between rendering chain
document the new context name argument
document the new [context <name>( message
add 2 new examples to explain multiple window system
add texture sharing between context example

this doesn't work (neither on intel GPU nor on Nvidia (driver 340))
enable context sharing to make examples/14.multiple_windows/03_texture_sharing.pd work

this work on Intel driver (	ubuntu 14.10)
rename example to follow default naming convention
change filename in Makefile.am
Merge remote-tracking branch 'avilleret/multiple_window'
implement "bang" and "render" msgs

this should allow to use single-buffered mode as it used to be.

Closes: https://github.com/umlaeute/Gem/issues/87