Copies the pixel data from a cogl texture to system memory.
<note>Don't pass the value of cogl_texture_get_rowstride as the rowstride argument, the rowstride should 
            be the rowstride you want for the destination data buffer not the rowstride of the source texture</note>
| this | a Texture pointer. | 
| format | the PixelFormat to store the texture as. | 
| rowstride | the rowstride of  | 
| data | memory location to write the this's contents, or null to only query the data size through the return value. | 
| the size of the texture data in bytes |