This adds a format parameter to screenshot-raw command which controls
the format of the returned image data. In addition to the bgr0 format,
this adds support for 3 other formats: bgra, rgba, rgba64.
bgra is used by overlay-add command and several window systems.
rgba and rgba64 are used by several GPU APIs and are the formats
returned by the gpu and gpu-next screenshot code, which avoid the need
for further format conversion (see convert_image function).
rgba64 also adds the ability to acquire high bit depth screenshots
for screenshot-raw command.
