gallium: add st_api feature mask to prevent advertising MS visuals

v2: use a define for the maximum sample count
v3: also test odd sample counts (r300 supports MS3)

While multisample renderbuffers are supported by mesa, MS visuals
are not, so we need a way to tell dri/st not to advertise them even
if the gallium driver does support multisampled surfaces.

Otherwise applications selecting these non-functional visuals would
run into trouble ...

Reviewed-by: Brian Paul <brianp@vmware.com>

src/gallium/include/state_tracker/st_api.h

@@ -69,6 +69,17 @@ enum st_profile_type
 #define ST_PROFILE_OPENGL_ES1_MASK   (1 << ST_PROFILE_OPENGL_ES1)
 #define ST_PROFILE_OPENGL_ES2_MASK   (1 << ST_PROFILE_OPENGL_ES2)
 
+/**
+ * Optional API/state tracker features.
+ */
+enum st_api_feature
+{
+   ST_API_FEATURE_MS_VISUALS  /**< support for multisample visuals */
+};
+
+/* for feature_mask in st_api */
+#define ST_API_FEATURE_MS_VISUALS_MASK (1 << ST_API_FEATURE_MS_VISUALS)
+
 /**
  * New context flags for GL 3.0 and beyond.
  *
@@ -429,6 +440,11 @@ struct st_api
     */
    unsigned profile_mask;
 
+   /**
+    * The supported optional features.  Tested with ST_FEATURE_*_MASK.
+    */
+   unsigned feature_mask;
+
    /**
     * Destroy the API.
     */