Minimal stopgaps for MCI to boot

- Clamp staging buffer size to 2GB to prevent Vulkan allocation failures
- Add size validation in MappedUploadMemory to avoid buffer overruns

src/video_core/buffer_cache/buffer_cache.h

@@ -1510,6 +1510,9 @@ void BufferCache<P>::MappedUploadMemory([[maybe_unused]] Buffer& buffer,
     if constexpr (USE_MEMORY_MAPS) {
         auto upload_staging = runtime.UploadStagingBuffer(total_size_bytes);
         const std::span<u8> staging_pointer = upload_staging.mapped_span;
+        if (staging_pointer.size() < total_size_bytes) {
+            return;
+        }
         for (BufferCopy& copy : copies) {
             u8* const src_pointer = staging_pointer.data() + copy.src_offset;
             const DAddr device_addr = buffer.CpuAddr() + copy.dst_offset;

src/video_core/renderer_vulkan/vk_staging_buffer_pool.cpp

@@ -181,7 +181,7 @@ std::optional<StagingBufferRef> StagingBufferPool::TryGetReservedBuffer(size_t s
 
 StagingBufferRef StagingBufferPool::CreateStagingBuffer(size_t size, MemoryUsage usage,
                                                         bool deferred) {
-    const u32 log2 = Common::Log2Ceil64(size);
+    const u32 log2 = (std::min)(Common::Log2Ceil64(size), 31U);
     VkBufferCreateInfo buffer_ci = {
         .sType = VK_STRUCTURE_TYPE_BUFFER_CREATE_INFO,
         .pNext = nullptr,