Vulkan Video Filters: HDR Tonemapping and Subtitles Rendering

VLC's Vulkan video output module is missing two features: HDR tonemapping and subtitle rendering. HDR content played on an SDR monitor appears washed out because no color mapping is applied. The project will be integrating libplacebo tonemapping into Vulkan's fragment shader, so HDR videos can be displayed correctly, by using libplacebo API to generate GLSL code at runtime, compiling it to SPIR-V using shaderc, and injecting it into the fragment shader. Also, currently, subtitles are not rendered at all in the video output module of Vulkan. The project will implement a dedicated Vulkan sub-renderer (similar to the OpenGL one) with alpha-blending enabled to upload the RGBA subtitle regions as textures, to be able to draw them over the video output in their correct positions.