Add HW context support to i915 driver

Starting with Gen6 (Sandy Bridge), Mesa 9.2+ relies on hardware context switching in the i915 driver.

We're insterested in test reports (success or failure) on any GPU (even non-Intel).

1. Install Mesa 10.3 from the "experimental" branch of the xorg-dev repository. You need to rebuild at least the following ports:
   • graphics/libdrm
   • graphics/libglapi
   • graphics/libGL
   • graphics/dri
2. Check if Mesa complains about missing hw context:

# glxinfo
name of display: :0.0
Gen6+ requires Kernel 3.6 or later.
Assertion failed: (ctx->Version > 0), function handle_first_current, file ../../src/mesa/main/context.c, line 1498.
Abort (core dumped)

1. Clone the Git branch or apply the patch to your local source tree
2. Build and install kernel
3. Reboot
4. Re-run glxinfo

# glxinfo
name of display: :0
display: :0  screen: 0
direct rendering: Yes
...
OpenGL renderer string: Mesa DRI Intel(R) 965GM 
OpenGL version string: 2.1 Mesa 10.2.4
...

1. Play with your usual GL applications.

Please report any success or failure by following the instructions on the Graphics portal.