Fix number of stencil bits when FBO is used

676ecab52ed4c7209620dddf01f1b7043db5a6a9 disabled both display and framebuffer stencil bits. `ForgeHooksClient.stencilBits` value, used also for [FBO stencil buffer allocation](https://github.com/MinecraftForge/MinecraftForge/blob/master/patches/minecraft/net/minecraft/client/shader/Framebuffer.java.patch#L7), is always set to 0, unless forces `forge.forceDisplayStencil`.

This PR adds extra logic to handle cases where display stencil buffer is disabled, but one can be allocated for FBO