D3D12: Stage BindGroups on CPU descriptor heaps.

Instead of directly populating GPU heaps, pre-encoded
BindGroups are staged on CPU heaps then copied over
to the GPU. Non-shader visible allocators are stored
on the BGL, which hands out fixed-size chunks to
simplify memory managment. To enable memory re-use,
CPU allocations are tied to the lifetime of BindGroup
objects.

BUG=dawn:155

Change-Id: I402e6686c96f7450a077c627c8499600979e426c
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/18100
Commit-Queue: Bryan Bernhart <bryan.bernhart@intel.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

src/common/Math.cpp

@@ -19,6 +19,7 @@
 
 #include <algorithm>
 #include <cmath>
+#include <limits>
 
 #if defined(DAWN_COMPILER_MSVC)
 #    include <intrin.h>
@@ -152,3 +153,10 @@ float SRGBToLinear(float srgb) {
         return std::pow((srgb + 0.055f) / 1.055f, 2.4f);
     }
 }
+
+uint64_t RoundUp(uint64_t n, uint64_t m) {
+    ASSERT(m > 0);
+    ASSERT(n > 0);
+    ASSERT(m <= std::numeric_limits<uint64_t>::max() - n);
+    return ((n + m - 1) / m) * m;
+}