VkPipelineCache: Do not publically expose internal mutexes

They make it unnecessarily easy to write code that violates the mutex lock requirements.

Replace with the `getOrCreate` pattern already used in vk::Device, which keeps the locking internal.

This change fixes a couple of places that we were accesssing data without correct locking (real bugs).

Bug: b/153194656
Change-Id: I132c450594f4042160b575197789bca7f1a5e25f
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43650
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>

src/Vulkan/VkDevice.hpp

diff --git a/src/Vulkan/VkDevice.hpp b/src/Vulkan/VkDevice.hpp
index 3ba442d..8931949 100644
--- a/src/Vulkan/VkDevice.hpp
+++ b/src/Vulkan/VkDevice.hpp
@@ -86,8 +86,14 @@
 			};
 		};
 
+		// getOrCreate() queries the cache for a Routine with the given key.
+		// If one is found, it is returned, otherwise createRoutine(key) is
+		// called, the returned Routine is added to the cache, and it is
+		// returned.
+		// Function must be a function of the signature:
+		//     std::shared_ptr<rr::Routine>(const Key &)
 		template<typename Function>
-		std::shared_ptr<rr::Routine> getOrCreate(const Key &key, Function createRoutine)
+		std::shared_ptr<rr::Routine> getOrCreate(const Key &key, Function &&createRoutine)
 		{
 			std::lock_guard<std::mutex> lock(mutex);