OpenGL: Improve frameclear

Make sure to clear the depth buffer, and the builtin color buffer when using an FBO.
Fixes issues with some VM drivers

code/midtown/agigl/glpipe.cpp

@@ -264,17 +264,29 @@ void agiGLPipeline::BeginFrame()
 
     gl_context_->MakeCurrent();
 
-    if (PARAM_frameclear.get_or(true))
+    bool frameclear = PARAM_frameclear.get_or(true);
+
+    if (frameclear)
     {
         agiGL->EnableDisable(GL_SCISSOR_TEST, false);
+        agiGL->DepthMask(true);
         glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
-        glClear(GL_COLOR_BUFFER_BIT);
     }
 
     if (fbo_ != 0)
     {
+        if (frameclear)
+        {
+            glClear(GL_COLOR_BUFFER_BIT);
+        }
+
         glBindFramebuffer(GL_FRAMEBUFFER, fbo_);
     }
+
+    if (frameclear)
+    {
+        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
+    }
 }
 
 void agiGLPipeline::BeginScene()