DRW: View: Add base for multi-view support

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.

source/blender/draw/intern/draw_command.cc

@@ -567,7 +567,9 @@ void DrawCommandBuf::bind(RecordingState &state,
 void DrawMultiBuf::bind(RecordingState &state,
                         Vector<Header, 0> &headers,
                         Vector<Undetermined, 0> &commands,
-                        VisibilityBuf &visibility_buf)
+                        VisibilityBuf &visibility_buf,
+                        int visibility_word_per_draw,
+                        int view_len)
 {
   UNUSED_VARS(headers, commands);
 
@@ -607,6 +609,8 @@ void DrawMultiBuf::bind(RecordingState &state,
     GPUShader *shader = DRW_shader_draw_command_generate_get();
     GPU_shader_bind(shader);
     GPU_shader_uniform_1i(shader, "prototype_len", prototype_count_);
+    GPU_shader_uniform_1i(shader, "visibility_word_per_draw", visibility_word_per_draw);
+    GPU_shader_uniform_1i(shader, "view_shift", log2_ceil_u(view_len));
     GPU_storagebuf_bind(group_buf_, GPU_shader_get_ssbo(shader, "group_buf"));
     GPU_storagebuf_bind(visibility_buf, GPU_shader_get_ssbo(shader, "visibility_buf"));
     GPU_storagebuf_bind(prototype_buf_, GPU_shader_get_ssbo(shader, "prototype_buf"));