mesa: Skip clearing color buffers when color writes are disabled.

WebGL Aquarium in Chrome 24 actually hits this.

v2: Move to core Mesa (wisely suggested by Ian); only consider
    components which actually exist.

v3: Use _mesa_format_has_color_component to determine whether components
    actually exist, fixing alpha format handling.

v4: Add a comment, as requested by Brian.  No actual code changes.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Tested-by: Dylan Baker <baker.dylan.c@gmail.com>

src/mesa/main/clear.c

@@ -106,6 +106,31 @@ _mesa_ClearColorIuiEXT(GLuint r, GLuint g, GLuint b, GLuint a)
 }
 
 
+/**
+ * Returns true if color writes are enabled for the given color attachment.
+ *
+ * Beyond checking ColorMask, this uses _mesa_format_has_color_component to
+ * ignore components that don't actually exist in the format (such as X in
+ * XRGB).
+ */
+static bool
+color_buffer_writes_enabled(const struct gl_context *ctx, unsigned idx)
+{
+   struct gl_renderbuffer *rb = ctx->DrawBuffer->_ColorDrawBuffers[idx];
+   GLuint c;
+   GLubyte colorMask = 0;
+
+   if (rb) {
+      for (c = 0; c < 4; c++) {
+         if (_mesa_format_has_color_component(rb->Format, c))
+            colorMask |= ctx->Color.ColorMask[idx][c];
+      }
+   }
+
+   return colorMask != 0;
+}
+
+
 /**
  * Clear buffers.
  * 
@@ -181,7 +206,7 @@ _mesa_Clear( GLbitfield mask )
          for (i = 0; i < ctx->DrawBuffer->_NumColorDrawBuffers; i++) {
             GLint buf = ctx->DrawBuffer->_ColorDrawBufferIndexes[i];
 
-            if (buf >= 0) {
+            if (buf >= 0 && color_buffer_writes_enabled(ctx, i)) {
                bufferMask |= 1 << buf;
             }
          }