Fix shader not working on macos

[Catizard]

This pr is demanding volunteers for testing on linux and widnows.

This pr aims to fix the issue that shander doesn't work on macos, and properly, linux. The initial problem is some old bms, e.g. gengaozo, is using an old trick which is so called "black color keying" to mimic the "transparent color". In beatoraja, this is achieved by using a shader. However, the shader file is using the old standard that simply won't compile on macos and linux. Causing gengaozo's bga looks funny and has crazy flashes that can burn ppl eyes.

In this pr, I rerouted the shader files to _unix variant if os is mac or linux.

For more information, please check out https://hitkey.bms.ms/cmds.htm#BMPXX-LAYER.

This pr has a high possibility is related to #210, I haven't looked into it yet

Side notes for devs
There was an old issue when supporting mac: the bga video is displaying RGB color in wrong order (Expected RGB, Got BGR iirc), this issue is having a high possibility that is because the ffmpeg shader is not applying because it cannot be compiled on macos

[Catizard]

After looking into the issues pointed in #210, I found none of them would be fixed in this pr although they seems to be rather related to the problem solved in this pr. Especially the transparent colors are still broken after the changes based on the fact that they are looking fine on my test application.