[graphics] Initialize shadows FB with a depth buffer

[modawan]

Always attach a depth buffer during initialization. Render calls glClear(GL_DEPTH_BUFFER_BIT) before a depth buffer is attached. If nothing is attached before glClear, some drivers trip and never recover.

Reproduced on Linux with Nvidia GTX 1070 and driver versions 535.261 and 550.163.

Fixes #82 "Retro renderer is broken".

[modawan]

The following patch also helps. I think the driver trips on glClear(GL_DEPTH_BUFFER_BIT) when there is no depth buffer attached, and never recovers. If we skip glClear for the first frame, the bug disappears.

diff --git a/include/reone/scene/render/pipeline/retro.h b/include/reone/scene/render/pipeline/retro.h
index d621a68d6..53cbd45f3 100644
--- a/include/reone/scene/render/pipeline/retro.h
+++ b/include/reone/scene/render/pipeline/retro.h
@@ -76,6 +76,7 @@ private:
     };
 
     RenderTargets _targets;
+    bool _shouldClear { false };
 
     void initRenderTargets();
 };
diff --git a/src/libs/scene/render/pipeline/retro.cpp b/src/libs/scene/render/pipeline/retro.cpp
index 9f0d893e3..b7bda0f49 100644
--- a/src/libs/scene/render/pipeline/retro.cpp
+++ b/src/libs/scene/render/pipeline/retro.cpp
@@ -176,7 +176,9 @@ Texture &RetroRenderPipeline::render() {
     if (dirLightShadows || pointLightShadows) {
         _context.bindDrawFramebuffer(*_targets.shadows, {});
         _context.withViewport(glm::ivec4 {0, 0, _options.shadowResolution, _options.shadowResolution}, [this, &pass, &dirLightShadows]() {
-            _context.clearDepth();
+            if (_shouldClear) {
+                _context.clearDepth();
+            }
             if (dirLightShadows) {
                 _targets.shadows->attachTexture(*_targets.dirLightShadowsDepth, Framebuffer::Attachment::Depth);
                 _passCallbacks.at(RenderPassName::DirLightShadowsPass)(pass);
@@ -184,6 +186,7 @@ Texture &RetroRenderPipeline::render() {
                 _targets.shadows->attachTexture(*_targets.pointLightShadowsDepth, Framebuffer::Attachment::Depth);
                 _passCallbacks.at(RenderPassName::PointLightShadows)(pass);
             }
+            _shouldClear = true;
         });
     }