Flag enables backends to differentiate between a framebuffer
bind with a custom loadstore state and a standard bind.
For Metal, this resolves an ambiguous complexity about loading
or clearing attachments by only flagging the first bind call as
explicit.
This means if a framebuffer is re-bound by a secondary code-path,
the re-started render-pass will not perform a secondary load. This
now allows explicit clear state to be specified on any attachment
type. Previously only memoryless attachments supported this.
To avoid further complexity, usage of`GPU_framebuffer_clear_* `
calls in conjunction with `GPU_framebuffer_bind_ex` will now
trigger an assertion failure.
Authored by Apple: Michael Parkin-White
Pull Request: https://projects.blender.org/blender/blender/pulls/115734
34c6e31332