VideoCommon: Abort texture prefetching on low RAM

There is no nice way to correctly "detect" the "used" memory, so we just say
we're fine to use 50% of the physical memory for custom textures.

This will fix out-of-memory crashes, but we still might run into swapping issues.

Source/Core/Common/MemoryUtil.h

@@ -16,6 +16,7 @@ void ReadProtectMemory(void* ptr, size_t size);
 void WriteProtectMemory(void* ptr, size_t size, bool executable = false);
 void UnWriteProtectMemory(void* ptr, size_t size, bool allowExecute = false);
 std::string MemUsage();
+size_t MemPhysical();
 
 void GuardMemoryMake(void* ptr, size_t size);
 void GuardMemoryUnmake(void* ptr, size_t size);