vgsp-gl: add sema_sph_bind_mask for slot-selective synth SPH binds

With sema_gl_sph_bind=0 the r1538 window carries only the client's two
slot-1 (VERTEX_B) binds; the DS then faults a0040800 (PS stage
SPH11|SPH18, stage5=0) because the PS program start is missing. Add
sema_sph_bind_mask (bit j = synth slot j) so the PS bind can be
re-enabled alone (0x20) while the vertex synth binds stay off; the
legacy sph_bind=1 behavior is byte-identical (j2/j5 both emitted).

r1541 (mask=0x20): the receipt fires (j5=1 x8) but stage5 stays zero and
the launch poisons at LOAD_CONSTANT_BUFFER (0x2390 <- 0xbf800000,
EC 0x03), so the j5 synth emission needs the r1456 selector/state fix
before this axis can be evaluated. Parameter default 0; evidence in
Handoff-vGPU-IO.md.

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