v3dv: document why we don't expose VK_EXT_scalar_block_layout

And since this is an optional feature in Vulkan 1.2, fill in the
corresponding feature query while we are at it.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14731>

src/broadcom/vulkan/v3dv_device.c

@@ -1088,6 +1088,15 @@ v3dv_GetPhysicalDeviceFeatures2(VkPhysicalDevice physicalDevice,
       .hostQueryReset = true,
       .uniformAndStorageBuffer8BitAccess = true,
       .uniformBufferStandardLayout = true,
+      /* V3D 4.2 wraps TMU vector accesses to 16-byte boundaries, so loads and
+       * stores of vectors that cross these boundaries would not work correcly
+       * with scalarBlockLayout and would need to be split into smaller vectors
+       * (and/or scalars) that don't cross these boundaries. For load/stores
+       * with dynamic offsets where we can't identify if the offset is
+       * problematic, we would always have to scalarize. Overall, this would
+       * not lead to best performance so let's just not support it.
+       */
+      .scalarBlockLayout = false,
       .storageBuffer8BitAccess = true,
       .storagePushConstant8 = true,
    };
@@ -1191,6 +1200,13 @@ v3dv_GetPhysicalDeviceFeatures2(VkPhysicalDevice physicalDevice,
          break;
       }
 
+      case VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_SCALAR_BLOCK_LAYOUT_FEATURES_EXT: {
+         VkPhysicalDeviceScalarBlockLayoutFeaturesEXT *features =
+            (void *) ext;
+         features->scalarBlockLayout = vk12.scalarBlockLayout;
+         break;
+      }
+
       case VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_UNIFORM_BUFFER_STANDARD_LAYOUT_FEATURES_KHR: {
          VkPhysicalDeviceUniformBufferStandardLayoutFeaturesKHR *features =
             (VkPhysicalDeviceUniformBufferStandardLayoutFeaturesKHR *)ext;