Cleanup: clarify naming for MLoopTri

The term `looptri` was used ambiguously for both single & arrays.
The term `tri` was also used, causing `tri->tri`.

Use terms:

- `looptris` for an array or when dealing with multiple items.
- `looptri` is used when dealing with a single item.
- `lt` for a single MLoopTri variables & arguments.

This was already a convention but not followed closely.

intern/dualcon/dualcon.h

@@ -26,7 +26,7 @@ typedef struct DualConInput {
   int co_stride;
   int totco;
 
-  DualConTri looptri;
+  DualConTri looptris;
   int tri_stride;
   int tottri;
 

intern/dualcon/intern/dualcon_c_api.cpp

@@ -22,7 +22,7 @@ static void veccopy(float dst[3], const float src[3])
 }
 
 #define GET_TRI(_mesh, _n) \
-  (*(DualConTri)(((char *)(_mesh)->looptri) + ((_n) * (_mesh)->tri_stride)))
+  (*(DualConTri)(((char *)(_mesh)->looptris) + ((_n) * (_mesh)->tri_stride)))
 
 #define GET_CO(_mesh, _n) (*(DualConCo)(((char *)(_mesh)->co) + ((_n) * (_mesh)->co_stride)))