vulkan: use vkCmdClearColorImage to clear texture

using the vulkan functionality probably leads to less bugs.
Until now it was assumed that any image was in the rgba8 format,
which is of course not correct. Clearing a texture to white or black
should now work for any format.

src/modules/graphics/vulkan/Texture.h

@@ -40,6 +40,9 @@ namespace love {
 				void copyBufferToImage(VkBuffer, VkImage, const Rect&);
 				void createTextureImageView();
 				void createTextureSampler();
+				void clear(bool white);
+
+				VkClearColorValue getClearValue(bool white);
 
 				graphics::Graphics* gfx;
 				VkDevice device;