GPU: optionally hide unknown GPUs (#1742)

The current default behavior of not hiding any GPUs is retained. The new
feature of hiding unknown or unrecognized GPUs can be enabled with
hide-type = "unknown".

src/modules/gpu/option.h

@@ -7,7 +7,8 @@
 
 typedef enum __attribute__((__packed__)) FFGPUType
 {
-    FF_GPU_TYPE_UNKNOWN,
+    FF_GPU_TYPE_NONE,      // Indicates no specific GPU type. Useful as a hide filter only.
+    FF_GPU_TYPE_UNKNOWN,   // Indicates an unknown or unrecognized GPU type.
     FF_GPU_TYPE_INTEGRATED,
     FF_GPU_TYPE_DISCRETE,
 } FFGPUType;