frameline GW: add 12 bits gearbox suport

src/gateware/cxp_frame_pipeline.py

@@ -401,7 +401,7 @@ class Frame_Header_Decoder(Module):
 
 class Custom_Pixel_Gearbox(Module):
     def __init__(self, size):
-        assert size in [8, 10, 16]
+        assert size in [8, 10, 12, 16]
 
         self.x_size = Signal(3*char_width)
 
@@ -485,12 +485,12 @@ class Custom_Pixel_Gearbox(Module):
                 }
                 # mod 16
                 self.sync += Case(self.x_size[:4], stb_cases)
-            # case 12:
-            #     stb_cases = {
-            #       5: extra_stb.eq(1),
-            #     }
-            #     # mod 8
-            #     self.sync += Case(self.x_size[:3], stb_cases)
+            case 12:
+                stb_cases = {
+                  5: extra_stb.eq(1),
+                }
+                # mod 8
+                self.sync += Case(self.x_size[:3], stb_cases)
 
 
         # NOTE: