Simplified BindGroup constructor

[mvaligursky]

• Simplified constructor of the BindGroup, which is manually specified for Compute shaders, to require a single array instead of up to 4 arrays.
• Slot allocation is also moved to platform independent level, to make it available and easy to understand to the user.

Before:

new pc.BindGroupFormat(device, [
        // no uniform buffer
    ], [
        new pc.BindTextureFormat('uSceneColorMap', pc.SHADERSTAGE_COMPUTE, undefined, undefined, false)
    ], [
        // no storage textures
    ], [
        new pc.BindStorageBufferFormat('outBuffer', pc.SHADERSTAGE_COMPUTE)
    ])
)

Now:

new pc.BindGroupFormat(device, [
    new pc.BindTextureFormat('uSceneColorMap', pc.SHADERSTAGE_COMPUTE, undefined, undefined, false),
    new pc.BindStorageBufferFormat('outBuffer', pc.SHADERSTAGE_COMPUTE)
])