I don't know for certain, but I suspect the answer is "you can't" for the reasons given in their reply to a user requesting OpenGL support. Just in case that URL changes/disappears, I'll quote the relevant bits below;
- Why does the PITFT not support OpenGL?
- Are there technical limitations that prevent the screen from supporting OpenGL?
Because it's not directly interfaced to the video circuitry; it's a
"dumb" framebuffer connected via the SPI bus on the GPIO header, and
only runs so fast. GL requires a high speed interface between the GPU
and display, which we don't have here.
- Will OpenGL be supported in future versions of hardware/firmware?
Not the PiTFT. If you're patient though, at some point the Raspberry
Pi Foundation will release their own display that connects via the
LVDS header. In theory anyway, they've talked about it. Perhaps if
it's well-documented others can offer displays in other sizes and
form-factors, but I have no idea if that aspect will be open and I'm
really talking out my rear at this point.
Another alternative is to use either the HDMI or composite interfaces
to connect a small external display, driven by the video circuitry and
thus GL compatible. Before the PiTFT I'd seen some other emulation
projects using such displays...the wiring isn't as tidy and the
composite displays won't be razor-sharp due to interpolation...but
there you go.
Also also, there are utilities that continually copy the main
framebuffer (under GPU control) to the secondary buffer used by the
PiTFT. The performance is really quite underwhelming though, and this
is why we didn't use that approach for the Cupcade project.