Move software renderer into its own namespace to isolate its globals and make

any access explicit. This reveals the places in the code where they are being
accessed where they shouldn't and prevents accidental usage.

src/r_drawt.cpp

@@ -47,6 +47,9 @@
 #include "r_things.h"
 #include "v_video.h"
 
+namespace swrenderer       
+{
+
 // I should have commented this stuff better.
 //
 // dc_temp is the buffer R_DrawColumnHoriz writes into.
@@ -1130,3 +1133,5 @@ void R_FillColumnHorizP_C (void)
 		dest += 8;
 	} while (--count);
 }
+
+}