core: GP_Pixel.gen.h.t: Add #define GP_PIXEL_foo

So that we can use:

 #ifdef GP_PIXEL_RGBA5551
	...
 #endif

for less common pixel types.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
gfxprim_config.py: Add RGB555.

This is needed to support 16 bit RGB BMP.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
loaders: BMP: Add support for RGB555.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>