Previously, this was done partially (for fixed array types only).

startIdx should be the first index to draw, either from the vertex
array or the index array, depending on if the draw is indexed or
not. Having both at the same time wouldn't make sense.

We want to know if this happens a lot. If the query is always called
from a different context than the one that created it, occlusion
culling would effectively be disabled, which could have a significant
performance impact, depending on the type of objects being culled.

In practice we don't implement WINED3DSAMP_DMAPOFFSET anyway, so this
is unlikely to make much of a difference.