meta: Add infrastructure for saving/restoring the DrawBuffers state.

Sometimes we need to configure what draw buffers we render to, without
creating a new FBO.  This path will make that possible.

Cc: "10.2" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>

src/mesa/drivers/common/meta.c

@@ -392,6 +392,24 @@ _mesa_meta_init(struct gl_context *ctx)
    ctx->Meta = CALLOC_STRUCT(gl_meta_state);
 }
 
+static GLenum
+gl_buffer_index_to_drawbuffers_enum(gl_buffer_index bufindex)
+{
+   assert(bufindex < BUFFER_COUNT);
+
+   if (bufindex >= BUFFER_COLOR0)
+      return GL_COLOR_ATTACHMENT0 + bufindex - BUFFER_COLOR0;
+   else if (bufindex == BUFFER_FRONT_LEFT)
+      return GL_FRONT_LEFT;
+   else if (bufindex == BUFFER_FRONT_RIGHT)
+      return GL_FRONT_RIGHT;
+   else if (bufindex == BUFFER_BACK_LEFT)
+      return GL_BACK_LEFT;
+   else if (bufindex == BUFFER_BACK_RIGHT)
+      return GL_BACK_RIGHT;
+
+   return GL_NONE;
+}
 
 /**
  * Free context meta-op state.
@@ -778,6 +796,23 @@ _mesa_meta_begin(struct gl_context *ctx, GLbitfield state)
          _mesa_set_framebuffer_srgb(ctx, GL_FALSE);
    }
 
+   if (state & MESA_META_DRAW_BUFFERS) {
+      int buf, real_color_buffers = 0;
+      memset(save->ColorDrawBuffers, 0, sizeof(save->ColorDrawBuffers));
+
+      for (buf = 0; buf < MAX_DRAW_BUFFERS; buf++) {
+         int buf_index = ctx->DrawBuffer->_ColorDrawBufferIndexes[buf];
+         if (buf_index == -1)
+            continue;
+
+         save->ColorDrawBuffers[buf] =
+            gl_buffer_index_to_drawbuffers_enum(buf_index);
+
+         if (++real_color_buffers >= ctx->DrawBuffer->_NumColorDrawBuffers)
+            break;
+      }
+   }
+
    /* misc */
    {
       save->Lighting = ctx->Light.Enabled;
@@ -1176,6 +1211,10 @@ _mesa_meta_end(struct gl_context *ctx)
        ctx->CurrentRenderbuffer->Name != save->RenderbufferName)
       _mesa_BindRenderbuffer(GL_RENDERBUFFER, save->RenderbufferName);
 
+   if (state & MESA_META_DRAW_BUFFERS) {
+      _mesa_DrawBuffers(MAX_DRAW_BUFFERS, save->ColorDrawBuffers);
+   }
+
    ctx->Meta->SaveStackDepth--;
 
    ctx->API = save->API;

src/mesa/drivers/common/meta.h

@@ -181,6 +181,9 @@ struct save_state
    GLboolean TransformFeedbackNeedsResume;
 
    GLuint DrawBufferName, ReadBufferName, RenderbufferName;
+
+   /** MESA_META_DRAW_BUFFERS */
+   GLenum ColorDrawBuffers[MAX_DRAW_BUFFERS];
 };
 
 /**