This implements the base needed for supporting multiple view concurently
inside the same drawcall.
The view used by common macros and view related functions is indexed using
a global variable `drw_view_id` which can be set arbitrarly or read
from the `drw_ResourceID`.
This is needed for EEVEE-Next shadow but can be used for other purpose
in the future.
Note that a shader specialization is needed for it to work. `DRW_VIEW_LEN`
needs to be defined to the amount of view the shader will access.
The number of views contained in a `draw::View` is set at construction
time.
Note that the maximum number of object correctly drawn by the shaders
using multiple views will be lower than thoses who don't.
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