Initial support for integer format textures.

They can't be rendered to and ImageData can't use the formats, for now.

src/modules/graphics/Texture.cpp

@@ -504,6 +504,9 @@ void Texture::generateMipmaps()
 	if (isPixelFormatDepthStencil(format))
 		throw love::Exception("generateMipmaps cannot be called on a depth/stencil Texture.");
 
+	if (isPixelFormatInteger(format))
+		throw love::Exception("generateMipmaps cannot be called on an integer Texture.");
+
 	generateMipmapsInternal();
 }