MAINT-7977: If getVertexStrider() returns false, abandon benchmark.

Ruslan tracked the observed crash to assignments (to create a dummy triangle)
through an LLStrider<LLVector3> obtained from getVertexStrider(). When
getVertexStrider() returns false, produce a warning and just skip the rest of
the benchmark test.

The one bit of explicit cleanup apparently required by that early exit is a
call to LLImageGL::deleteTextures() to match the preceding generateTextures()
call. Wrap both in a new TextureHolder class whose destructor takes care of
cleanup. The only other references to the corresponding U32 array are a couple
calls to LLTexUnit::bindManual(); add a bind() method to support that.

Also fix apparent bug in the LL_DARWIN special case for "improbably high and
likely incorrect": the code assigned -1.f (the "couldn't compute" value) to
gbps, overlooking the fact that gbps is unconditionally recomputed below. In
the "likely incorrect" stanza, simply return -1.f instead.