glamor: Disable complex shaders on i915
The new Gallium based i915 Mesa driver for Intel's Gen3 GPUs is capable to run glamor, but the hardware has limited resources to link shaders. The gradient shader uses control flow, which is not supported. The rel_sampler function in the compositing shader is too complex to run on this device.
Eliminate the requirement of 128 native ALU instructions, and add i915 as driver-specific hack to skip these complex shaders.
Tested on an actual hardware (GMA 3100 with Q35 [Bearlake] chipset) both with Xwayland and with the Xorg modesetting driver, and the performance is pretty good.