Gfx::opSetFillGray: Make sure the colorspace is gray

Otherwise we will end up doing an uninitialized memory read down the
road

oss-fuzz/10040
7 jobs for 10040 in 5 minutes and 41 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #6554539
build

00:05:19

passed #6554544
build_android

00:00:39

passed #6554540
build_clang_libcpp

00:05:40

passed #6554543
build_clazy

00:04:54

passed #6554542
build_mingw64_fedora

00:02:55

passed #6554541
build_ubuntu_16_04

00:01:45

passed #6554538
clang_format

00:01:15