glx: Delete support for GLX_OML_swap_method.

This was an ill-advised extension.  While we advertised SWAP_COPY support,
we might implement it with a back-copy from the front buffer.  And we
never advertised EXCHANGE because we couldn't guarantee it.  So, if you
actually used this extension to try to reduce app redraws of the back
buffer, you might actually increase the bandwidth you used.  Whoops.

Instead, GLX_EXT_buffer_age and the similar EGL extension give you
feedback on what's left in your back buffer, letting you do minimum
redraws.

This reduces our GLX visual+fbconfig count from 1410 to 940 on an llvmpipe
X server.  Reducing visual counts will improve test runtime for
visual-iterating tests like piglit's glx-visuals-*.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25650>

include/GL/internal/dri_interface.h

@@ -742,7 +742,7 @@ struct __DRIuseInvalidateExtensionRec {
 #define __DRI_ATTRIB_OPTIMAL_PBUFFER_WIDTH	37
 #define __DRI_ATTRIB_OPTIMAL_PBUFFER_HEIGHT	38
 #define __DRI_ATTRIB_VISUAL_SELECT_GROUP	39
-#define __DRI_ATTRIB_SWAP_METHOD		40
+#define __DRI_ATTRIB_SWAP_METHOD		40 // Parsed by the X server when our visuals return it as an attrib.
 #define __DRI_ATTRIB_MAX_SWAP_INTERVAL		41
 #define __DRI_ATTRIB_MIN_SWAP_INTERVAL		42
 #define __DRI_ATTRIB_BIND_TO_TEXTURE_RGB	43
@@ -782,8 +782,8 @@ struct __DRIuseInvalidateExtensionRec {
 /* Note that with the exception of __DRI_ATTRIB_SWAP_NONE, we need to define
  * the same tokens as GLX. This is because old and current X servers will
  * transmit the driconf value grabbed from the AIGLX driver untranslated as
- * the GLX fbconfig value. __DRI_ATTRIB_SWAP_NONE is only used by dri drivers
- * to signal to the dri core that the driconfig is single-buffer.
+ * the GLX fbconfig value. These defines are kept for X Server suorce compatibility,
+ * since Mesa no longer exposes GLX_OML_swap_method.
  */
 #define __DRI_ATTRIB_SWAP_NONE                  0x0000
 #define __DRI_ATTRIB_SWAP_EXCHANGE              0x8061