EGL_KHR_platform_wayland: clarify EGLSurface locking (#160)

When querying EGLSurface for EGL_KHR_BUFFER_AGE or rendering to it
it will resuilt in locking its back buffer until the next swap.
This behavior is long established in mesa and is intentional.

The rationale behind this behavior is to jarring partial content
that could be if mesa throwed the back buffer in the middle of rendering
due to resize.

This commit explicitly states this behavior, so it won't be
surprising for the end users and also adds a dependency on
EGL_EXT_buffer_age.

extensions/KHR/EGL_KHR_platform_wayland.txt

@@ -40,6 +40,9 @@ Dependencies
     This extension is written against the EGL 1.5 Specification (draft
     20140122).
 
+    The behavior of part of this extension is different depending on whether
+    the EGL_EXT_buffer_age extension is also present.
+
 Overview
 
     This extension defines how to create EGL resources from native Wayland
@@ -85,6 +88,11 @@ New Behavior
     belongs to Wayland. Any such call fails and generates an
     EGL_BAD_PARAMETER error.
 
+    Rendering to the obtained EGLSurface or querying it with EGL_BUFFER_AGE_KHR
+    will lock its back buffer preventing it from being dropped or resized,
+    until the next buffer swap. The rationale behind this behavior is to keep
+    operations result accurate until the next swap.
+
 Issues
 
     1. Should this extension permit EGL_DEFAULT_DISPLAY as input to
@@ -99,6 +107,10 @@ Issues
        RESOLVED. No. Wayland has no pixmap type.
 
 Revision History
+    Version 3, 2022/07/14 (Kirill Chibisov)
+        - Clarify EGLSurface back buffer locking behavior with regards to
+          rendering and surface querying operations.
+        - Add dependency on EGL_EXT_buffer_age.
 
     Version 2, 2014/02/18 (Chad Versace)
         - Change resolution of issue #1 from "no" to "yes". Now