largely a merge of the previously discussed origin/gallium-resource-sampling
but updated.
the idea is to allow arbitrary binding of resources, the way opencl, new gl
versions and dx10+ require, i.e.
    DCL RES[0], 2D, FLOAT

    LOAD DST[0], SRC[0], RES[0]
    SAMPLE DST[0], SRC[0], RES[0], SAMP[0]