texture_cache: keep the allocator's block slack out of the usable VRAM

The collector's usage leaves out the unused part of VMA's blocks, but
the driver counts it and a heap at its budget cannot give it to a new
image. On an RX 7800 XT that slack was 1.06 GiB, the whole 1 GiB margin:
the heap sat at its budget for a minute while the collector saw room,
then images stopped fitting and the process died. Reserve the slack
(capped at 1.5 GiB) on top of the margin: at once when it grows, slowly
when it shrinks, so eviction leaving partly used blocks doesn't chase
the threshold down.

Assisted-by: Claude Sonnet 5.5 (diagnosis and code)
(cherry picked from commit 73fd5a489ea28226fa16a55f5008e622d631b54c)

1 files changed, 10 insertions, 0 deletions

diff --git a/src/video_core/vulkan_common/vulkan_device.cpp b/src/video_core/vulkan_common/vulkan_device.cpp
index 2ecbd2cb94..01e13cb384 100644
--- a/src/video_core/vulkan_common/vulkan_device.cpp
+++ b/src/video_core/vulkan_common/vulkan_device.cpp
@@ -1684,6 +1684,16 @@ u64 Device::GetDeviceMemoryUsage() const {
     return result;
 }
 
+u64 Device::GetDeviceMemorySlack() const {
+    if (allocator == nullptr) {
+        return 0;
+    }
+    std::array<VmaBudget, VK_MAX_MEMORY_HEAPS> heap_budgets{};
+    vmaGetHeapBudgets(allocator, heap_budgets.data());
+    const VmaStatistics& statistics = heap_budgets[main_heap].statistics;
+    return statistics.blockBytes - (std::min)(statistics.blockBytes, statistics.allocationBytes);
+}
+
 u64 Device::GetDeviceMemoryBudget() const {
     if (!extensions.memory_budget) {
         return 0;