1. A method comprising: determining, with a graphics processor, to operate in a first shader program execution mode; in response to the determination to operate in the first shader program execution mode, executing, with a shader unit of the graphics processor, a first plurality of instances of a merged vertex/geometry shader program that performs vertex shader processing and geometry shader processing, so that a single instance of the merged vertex/geometry shader program generates a first plurality of output vertices for each of a first plurality of primitives received by the merged vertex/geometry shader program; determining, with the graphics processor, to operate in a second shader program execution mode; and in response to the determination to operate in the second shader program execution mode, executing, with the shader unit of the graphics processor, a second plurality of instances of the merged vertex/geometry shader program to generate a second plurality of output vertices for each of a second plurality of primitives, so that each instance of the second plurality of instances of the merged vertex/geometry shader program outputs one output vertex.