LibPDF: Sketch out DeviceN color spaces a bit

Documents using them now show render-time diagnostics instead
of asserting that number of parameters passed to a color don't
match whatever number of channels the previously-set color space
had.

Fixes two asserts on the `-n 500` 0000.zip test set.

Userland/Libraries/LibPDF/CommonNames.h

@@ -51,6 +51,7 @@
     X(Dests)                      \
     X(DeviceCMYK)                 \
     X(DeviceGray)                 \
+    X(DeviceN)                    \
     X(DeviceRGB)                  \
     X(Differences)                \
     X(E)                          \