fix(bgfx) Change parameters for is_frame_buffer_valid

[bonenaut7]

BGFX's method bgfx::isFrameBufferValid takes array size, and array reference of Attachment's.
However in the bindings there is only single BGFXAttachment object, instead of BGFXAttachment.Buffer as it's done in the bgfx_create_frame_buffer_from_attachment.

I'm replaced MapToInt with AutoSize, now is_frame_buffer_valid has BGFXAttachment.Buffer and same buffer could be utilized both for validating attachments and creating framebuffer from attachments.

[Spasi]

Thank you @bonenaut7!