multicontext: [gemglutwindow] crashes on destroy

[umlaeute]

with multiple [gemglutwindow]s open, sending a [destroy( message to any of them crashes Pd.

This is on Debian, with freeglut_3.4.0-1, and the Debian gem package

[...]
X Error of failed request:  BadMatch (invalid parameter attributes)
  Major opcode of failed request:  152 (GLX)
  Minor opcode of failed request:  11 (X_GLXSwapBuffers)
  Serial number of failed request:  7878
  Current serial number in output stream:  7878
user@host$ echo $?
1

[umlaeute]

hmm, not always.

the problem seems to be, that the GLCONTEXT is gone and no longer valid. most likely the fix is to just properly implement gem::Context::push() to use a stack-like structure.