Provide a common way for Prism to call down to the platform to find out how much VRAM is available for textures (not including VRAM already allocated for other purposes)

The function should return the following:
 - 0: No platform-specific data: in this case Prism should check a system property "prism.vram" for the number of MBytes of VRAM available. If this property is not set, there is no limit.
 - A VRAM size in MBytes obtained from the platform.

The system property "prism.vram" should then be set to the value obtained (or zero in the "no limit" case)

Desktop platforms should all return 0 for "no platform specific data".
Embedded platforms should return a value obtained from the native libraries where possible, or 0 if no data is available.