fix for own regression [#34096] ocean sim vertex color bug

Caused by my assumption that alpha wasn't used for vertex colors.

Infact it is used by blender-internal rendering, but typically only for blending strand particles.
Updated comments to note this.

source/blender/makesdna/DNA_meshdata_types.h

@@ -118,9 +118,14 @@ typedef struct MLoopUV {
 #define MLOOPUV_VERTSEL	2
 #define MLOOPUV_PINNED	4
 
-/* at the moment alpha is abused for vertex painting
- * and not used for transparency,
- * note that red and blue are _not_ swapped, as they are with #MCol */
+/**
+ * at the moment alpha is abused for vertex painting,
+ * otherwise it should _always_ be initialized to 255
+ * Mostly its not used for transparency...
+ * (except for blender-internal rendering, see [#34096]).
+ *
+ * \note red and blue are _not_ swapped, as they are with #MCol
+ */
 typedef struct MLoopCol {
 	char r, g, b, a;
 } MLoopCol;

source/blender/modifiers/intern/MOD_ocean.c

@@ -486,7 +486,8 @@ static DerivedMesh *doOcean(ModifierData *md, Object *ob,
 
 						mlcol = &mloopcols[mp->loopstart + j];
 						mlcol->r = mlcol->g = mlcol->b = (char)(foam * 255);
-						/* mc->a = 255; */ /* no need to set */
+						/* This needs to be set (render engine uses) */
+						mlcol->a = 255;
 					} while (j--);
 				}
 			}