1. A method of operating a graphics processor that implements a tile-based graphics processing pipeline, the graphics processor comprising: a plurality of processing stages, including at least a rasteriser that rasterises input primitives to generate graphics fragments to be processed, each graphics fragment having one or more sampling points associated with it, and a renderer that processes fragments generated by the rasteriser to generate rendered fragment data; a local tile buffer configured to store rendered fragment data locally to the graphics processor prior to that rendered fragment data being written out from the graphics processor to an external memory; a write out stage configured to write rendered fragment data stored in the local tile buffer out from the graphics processor to an external memory; and a programmable tile shader stage operable to, in response to one or more executable graphics shader program instructions, read rendered fragment data stored in the local tile buffer, execute one or more graphics shader program instructions to perform a deferred shading operation using the read rendered fragment data, and write the result of the deferred shading operation into the local tile buffer or out from the graphics processor to an external memory; the method comprising: rendering plural render targets for a tile, the plural render targets containing rendered fragment data to be used in a deferred shading operation, and storing the plural render targets for the tile as distinct render targets in the local tile buffer, the plural render targets for the tile being accommodated simultaneously in the local tile buffer, wherein rendering the plural render targets comprises the renderer processing fragments generated by the rasteriser to generate rendered fragment data to be used in the deferred shading operation and wherein storing the plural render targets comprises storing rendered fragment data to be used in the deferred shading operation in the local tile buffer; and the programmable tile shader stage, under the control of one or more executable graphics shader program instructions to perform a deferred shading operation, reading rendered fragment data from two or more render targets of the plural render targets accommodated simultaneously in the local tile buffer, executing one or more graphics shader program instructions to perform the deferred shading operation using the read rendered fragment data from the two or more render targets of the plural render targets accommodated simultaneously in the local tile buffer, and writing the result of the deferred shading operation to an output render target in the local tile buffer, or out from the graphics processor to external memory.