Merge pull request #13922 from hrydgard/prevent-negative-vp-size

Make sure we don't try to set a negative viewport size.

GPU/Common/GPUStateUtils.cpp

@@ -575,6 +575,9 @@ void ConvertViewportAndScissor(bool useBufferedRendering, float renderWidth, flo
 		renderHeightFactor = renderHeight / 272.0f;
 	}
 
+	_assert_(renderWidthFactor > 0.0);
+	_assert_(renderHeightFactor > 0.0);
+
 	renderX = gstate_c.curRTOffsetX;
 
 	// Scissor
@@ -694,8 +697,11 @@ void ConvertViewportAndScissor(bool useBufferedRendering, float renderWidth, flo
 
 		out.viewportX = left * renderWidthFactor + displayOffsetX;
 		out.viewportY = top * renderHeightFactor + displayOffsetY;
-		out.viewportW = (right - left) * renderWidthFactor;
-		out.viewportH = (bottom - top) * renderHeightFactor;
+
+		// The calculations should end up with zero or positive values, but let's protect against any
+		// precision issues. See #13921.
+		out.viewportW = std::max(0.0f, (right - left) * renderWidthFactor);
+		out.viewportH = std::max(0.0f, (bottom - top) * renderHeightFactor);
 
 		// The depth viewport parameters are the same, but we handle it a bit differently.
 		// When clipping is enabled, depth is clamped to [0, 65535].  And minz/maxz discard.