vbo: reduce number of vertex buffer mappings for vertex attributes

Whenever we got a glColor, glNormal, glTexCoord, etc. call outside a
glBegin/End pair, we'd immediately map a vertex buffer to begin
accumulating vertex data.  In some cases, such as with display lists,
this led to excessive vertex buffer mapping.  For example, if we have
a display list such as:

glNewList(42, GL_COMPILE);
glBegin(prim);
glVertex2f();
...
glVertex2f();
glEnd();
glEndList();

Then did:

glColor3f();
glCallList(42);

We'd map a vertex buffer as soon as we saw glColor3f but we'd never
actually write anything to it.  Note that the vertex position data
was put into a vertex buffer during display list compilation.

With this change, we delay mapping the vertex buffer until we actually
have a vertex to write to it (triggered by a glVertex() call).  In the
above case, we no longer map a vertex buffer when setting the color and
calling the list.

For drivers such as VMware's, reducing buffer mappings gives improved
performance.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/mesa/vbo/vbo_exec_api.c

@@ -446,10 +446,6 @@ do {									\
                                                                         \
    assert(sz == 1 || sz == 2);                                          \
                                                                         \
-   if (unlikely(!(ctx->Driver.NeedFlush & FLUSH_UPDATE_CURRENT))) {     \
-      vbo_exec_begin_vertices(ctx);					\
-   }									\
-                                                                        \
    /* check if attribute size or type is changing */                    \
    if (unlikely(exec->vtx.active_sz[A] != N * sz) ||                    \
        unlikely(exec->vtx.attrtype[A] != T)) {                          \
@@ -470,6 +466,15 @@ do {									\
       /* This is a glVertex call */					\
       GLuint i;								\
 									\
+      if (unlikely((ctx->Driver.NeedFlush & FLUSH_UPDATE_CURRENT) == 0)) { \
+         vbo_exec_begin_vertices(ctx);                                  \
+      }                                                                 \
+                                                                        \
+      if (unlikely(!exec->vtx.buffer_ptr)) {                            \
+         vbo_exec_vtx_map(exec);                                        \
+      }                                                                 \
+      assert(exec->vtx.buffer_ptr);                                     \
+                                                                        \
       /* copy 32-bit words */                                           \
       for (i = 0; i < exec->vtx.vertex_size; i++)			\
 	 exec->vtx.buffer_ptr[i] = exec->vtx.vertex[i];			\
@@ -482,7 +487,10 @@ do {									\
 									\
       if (++exec->vtx.vert_count >= exec->vtx.max_vert)			\
 	 vbo_exec_vtx_wrap( exec );					\
-   }									\
+   } else {                                                             \
+      /* we now have accumulated per-vertex attributes */               \
+      ctx->Driver.NeedFlush |= FLUSH_UPDATE_CURRENT;                    \
+   }                                                                    \
 } while (0)
 
 #define ERROR(err) _mesa_error( ctx, err, __func__ )