Only copy dirty depth buffers between FBOs.

We can remove this if/when we track them separately.  This may break a
game that depends on the depth carrying over between several FBOs, but
that's not extremely likely.

This improves performance in Gods Eater Burst.

GPU/GLES/Framebuffer.cpp

@@ -744,7 +744,8 @@ void FramebufferManager::SetRenderFrameBuffer() {
 		vfb->dirtyAfterDisplay = true;
 		if ((gstate_c.skipDrawReason & SKIPDRAW_SKIPFRAME) == 0)
 			vfb->reallyDirtyAfterDisplay = true;
-		vfb->memoryUpdated = false; 
+		vfb->memoryUpdated = false;
+		vfb->depthUpdated = false;
 
 		if (g_Config.bTrueColor) {
 			vfb->colorDepth = FBO_8888;
@@ -907,7 +908,13 @@ void FramebufferManager::BindFramebufferDepth(VirtualFramebuffer *sourceframebuf
 	if (!sourceframebuffer || !targetframebuffer->fbo || !useBufferedRendering_) {
 		return;
 	}
-	
+
+	// If depth wasn't updated, then we're at least "two degrees" away from the data.
+	// This is an optimization: it probably doesn't need to be copied in this case.
+	if (!sourceframebuffer->depthUpdated) {
+		return;
+	}
+
 	if (MaskedEqual(sourceframebuffer->z_address, targetframebuffer->z_address) && 
 		sourceframebuffer->renderWidth == targetframebuffer->renderWidth &&
 		sourceframebuffer->renderHeight == targetframebuffer->renderHeight) {
@@ -923,6 +930,7 @@ void FramebufferManager::BindFramebufferDepth(VirtualFramebuffer *sourceframebuf
 			if (!gstate.isModeClear() || !gstate.isClearModeDepthMask()) {
 				fbo_bind_for_read(sourceframebuffer->fbo);
 				glBlitFramebuffer(0, 0, sourceframebuffer->renderWidth, sourceframebuffer->renderHeight, 0, 0, targetframebuffer->renderWidth, targetframebuffer->renderHeight, GL_DEPTH_BUFFER_BIT, GL_NEAREST);
+				// If we set targetframebuffer->depthUpdated here, our optimization above would be pointless.
 			}
 #endif
 		}

GPU/GLES/Framebuffer.h

@@ -58,6 +58,7 @@ struct VirtualFramebuffer {
 	int last_frame_used;
 	int last_frame_render;
 	bool memoryUpdated;
+	bool depthUpdated;
 
 	u32 fb_address;
 	u32 z_address;
@@ -174,6 +175,12 @@ public:
 		return displayFramebuf_ ? (0x04000000 | displayFramebuf_->fb_address) : 0;
 	}
 
+	void SetDepthUpdated() {
+		if (currentRenderVfb_) {
+			currentRenderVfb_->depthUpdated = true;
+		}
+	}
+
 	void NotifyFramebufferCopy(u32 src, u32 dest, int size);
 
 	void DestroyFramebuf(VirtualFramebuffer *vfb);

GPU/GLES/StateMapping.cpp

@@ -356,6 +356,9 @@ void TransformDrawEngine::ApplyDrawState(int prim) {
 		glstate.depthTest.enable();
 		glstate.depthFunc.set(GL_ALWAYS);
 		glstate.depthWrite.set(gstate.isClearModeDepthMask() || alwaysDepthWrite ? GL_TRUE : GL_FALSE);
+		if (gstate.isClearModeDepthMask() || alwaysDepthWrite) {
+			framebufferManager_->SetDepthUpdated();
+		}
 
 		// Color Test
 		bool colorMask = gstate.isClearModeColorMask();
@@ -398,6 +401,7 @@ void TransformDrawEngine::ApplyDrawState(int prim) {
 			glstate.depthTest.enable();
 			glstate.depthFunc.set(ztests[gstate.getDepthTestFunction()]);
 			glstate.depthWrite.set(gstate.isDepthWriteEnabled() || alwaysDepthWrite ? GL_TRUE : GL_FALSE);
+			framebufferManager_->SetDepthUpdated();
 		} else {
 			glstate.depthTest.disable();
 		}