Update doc and todo

Conveniently postpone the sync (fence/event/...) work until we actually
have a proper use case for it.

src/rhi/qrhi.cpp

@@ -242,6 +242,13 @@ QT_BEGIN_NAMESPACE
     QRhiResourceSharingHost. Resources can also outlive their creating QRhi in
     this case, as long as there is at least one QRhi left in the sharing group.
 
+    \note Support for efficient concurrent access to shared resources from QRhi
+    instances operating on different threads, especially when writing (for
+    example, rendering to a texture) is involved, is limited for the time
+    being. Neither GPU-GPU nor GPU-host synchronization facilities (such as,
+    fences or events) are provided in the QRhi API at the moment. These may be
+    introduced in the future.
+
     \sa {Qt Shader Tools}, QRhiResourceSharingHost
  */
 

todo.txt

-res.sh.: example to show read/write same texture
+res.sh.: example to show read/write same texture?
 event/fence for res.sh.?
 advanced blend modes
 gl: tex formats (texture, readback)
@@ -21,8 +21,6 @@ gl: ubuf structs, arrays
 test cubemap face as target
 test cubemap face readback
 object names for other than buf/rb/tex
-
-optional:
 compute?
 mtl: drawable warning?
 gl: target QOpenGLWindow/Widget?
@@ -41,8 +39,6 @@ multi-buffer (region) readback?
 depth readback?
 copy image depth?
 gl: markers and object names via gl_khr_debug
-
-shadertools:
 dxc for d3d as an alternative to fxc?
 hlsl -> dxc -> spirv -> spirv-cross hmmm...