1. No UI in video: end render pass before ImGui, capture, start new
render pass with loadOp=LOAD for ImGui, end again. Video captures
only the 3D scene.
2. Acid colors fixed: swapchain format changed from B8G8R8A8_SRGB to
B8G8R8A8_UNORM. SRGB format stores gamma-compressed values that
vkCmdCopyImageToBuffer reads raw — FFmpeg treated them as linear
causing acid colors. UNORM stores linear values, copy gives linear
data, FFmpeg bgra→yuv420p conversion is correct.
emil28092005
d6939e9efa