Fix textoverlay demo to use correct allocation size.
Fix textoverlay demo to use correct allocation size.

textoverlay was previously using a custom allocationSize for the font
texture. Changed that to use vkGetImageMemoryRequirements.

jherico/Vulkan,dreadwords/Vulkan,martty/Vulkan,SaschaWillems/Vulkan,ming4883/Vulkan,jpbruyere/Vulkan,jpbruyere/Vulkan,grim210/Vulkan,SaschaWillems/Vulkan,grim210/Vulkan,SaschaWillems/Vulkan,jherico/Vulkan,SaschaWillems/Vulkan,ming4883/Vulkan,SaschaWillems/Vulkan,ming4883/Vulkan