- change the images to be upside down until presentation to increase compatibility with shaders designed for OpenGL

- clamp scissors fully to avoid NVidia's awful drivers locking up the entire system if they end up out of bounds
- perform buffer clears as part of the render pass. this puts some restrictions on how FRenderState.Clear can be used
- add an offset uniform to the present shaders so the vulkan target can flip the image during presentation

src/rendering/gl/renderer/gl_postprocess.cpp

@@ -248,6 +248,7 @@ void FGLRenderer::DrawPresentTexture(const IntRect &box, bool applyGamma)
 		mPresentShader->Uniforms->ColorScale = (gl_dither_bpc == -1) ? 255.0f : (float)((1 << gl_dither_bpc) - 1);
 	}
 	mPresentShader->Uniforms->Scale = { screen->mScreenViewport.width / (float)mBuffers->GetWidth(), screen->mScreenViewport.height / (float)mBuffers->GetHeight() };
+	mPresentShader->Uniforms->Offset = { 0.0f, 0.0f };
 	mPresentShader->Uniforms.Set();
 	RenderScreenQuad();
 }