LibPDF: Allow /Pattern to be used directly as a color space name

Per spec:

"If the color space is one that can be specified by a name and no
additional parameters (DeviceGray, DeviceRGB, DeviceCMYK, and certain
cases of Pattern), the name may be specified directly."

We still don't implement /Pattern color spaces, but now we no longer
crash trying to look up the potentially-nonexistent /ColorSpace
dictionary on the page object when /Pattern is used directly as color
space name.

On top of #21514, reduces number of crashes on 300 random PDFs from the
web (the first 300 from 0000.zip from
https://pdfa.org/new-large-scale-pdf-corpus-now-publicly-available/)
from 42 (14%) to 34 (11%).

Userland/Libraries/LibPDF/ColorSpace.h

@@ -21,7 +21,7 @@
     V(Lab, false)                         \
     V(ICCBased, false)                    \
     V(Indexed, false)                     \
-    V(Pattern, false)                     \
+    V(Pattern, true)                      \
     V(Separation, false)                  \
     V(DeviceN, false)