Gawain: new immSkipAttrib function

Now you can explicitly skip a vertex attribute -- you don't give it a value and it won't get a copy of the previous vert's value. Useful for flat interpolated per-primitive values.

This is an advanced feature. Expect garbage in the empty spaces, and copies of garbage if you rely on the attrib copy behavior after skipping.

source/blender/gpu/gawain/immediate.c

@@ -592,6 +592,17 @@ void immAttrib4ubv(unsigned attrib_id, const unsigned char data[4])
 	immAttrib4ub(attrib_id, data[0], data[1], data[2], data[3]);
 	}
 
+void immSkipAttrib(unsigned attrib_id)
+	{
+#if TRUST_NO_ONE
+	assert(attrib_id < imm.vertex_format.attrib_ct);
+	assert(imm.vertex_idx < imm.vertex_ct);
+	assert(imm.primitive != PRIM_NONE); // make sure we're between a Begin/End pair
+#endif
+
+	setAttribValueBit(attrib_id);
+	}
+
 static void immEndVertex(void) // and move on to the next vertex
 	{
 #if TRUST_NO_ONE
@@ -619,6 +630,7 @@ static void immEndVertex(void) // and move on to the next vertex
 
 				GLubyte* data = imm.vertex_data + a->offset;
 				memcpy(data, data - imm.vertex_format.stride, a->sz);
+				// TODO: consolidate copy of adjacent attributes
 				}
 			}
 		}

source/blender/gpu/gawain/immediate.h

@@ -54,6 +54,10 @@ void immAttrib4ub(unsigned attrib_id, unsigned char r, unsigned char g, unsigned
 void immAttrib3ubv(unsigned attrib_id, const unsigned char data[4]);
 void immAttrib4ubv(unsigned attrib_id, const unsigned char data[4]);
 
+// explicitly skip an attribute
+// this advanced option kills automatic value copying for this attrib_id
+void immSkipAttrib(unsigned attrib_id);
+
 
 // provide one last attribute value & end the current vertex
 // this is most often used for 2D or 3D position (similar to glVertex)