Skip grungy_pages update when mark state invalid to speedup read_dirty

Skip grungy_pages update when mark state invalid to speedup read_dirty

* include/private/gc_priv.h [!GC_DISABLE_INCREMENTAL] (GC_read_dirty):
Add output_unneeded argument; update comment.
* os_dep.c [MANUAL_VDB || PROC_VDB] (GC_read_dirty): Likewise.
* mark.c [!GC_DISABLE_INCREMENTAL] (GC_initiate_gc): Pass
output_unneeded argument.
* misc.c [!GC_DISABLE_INCREMENTAL && !KEEP_BACK_PTRS]
(GC_enable_incremental): Likewise.
* os_dep.c [MPROTECT_VDB] (GC_read_dirty): Likewise.
* os_dep.c [GWW_VDB] (GC_gww_read_dirty): Add output_unneeded argument.
* os_dep.c [DEFAULT_VDB || PCR_VDB] (GC_read_dirty): Likewise.
* os_dep.c [GWW_VDB] (GC_gww_read_dirty): Do not call
BZERO(GC_grungy_pages) and set_pht_entry_from_index() if output_unneeded.
* os_dep.c [GWW_VDB && CHECKSUMS] (GC_gww_read_dirty): Add assertion
that output_unneeded is false.
* os_dep.c [MANUAL_VDB || MPROTECT_VDB] (GC_read_dirty): Do not call
BCOPY(GC_dirty_pages, GC_grungy_pages) if output_unneeded.
* os_dep.c [PROC_VDB] (GC_read_dirty): Do not call
memset(GC_grungy_pages) if output_unneeded.