In GL, OpenGL does the handling of texture priorities and swapping stuff, BlitzMax doesn't have any say in the matter.
The whole reason for having pixmaps initially, is that BlitzMax originally was based on OpenGL only and the OpenGL specification requires that the textures be in video ram in order to use them. Since it has features then to move graphics data to and from main memory, it refers to images in main memory as pixmaps (pixel maps). Textures are uploaded from pixmaps to textures and vice versa can be downloaded to a pixmap or the pixmap can be drawn directly to the backbuffer with glDrawPixels(). I think BRL also has other uses for pixmaps such as needing them in order to load in images from disk and for some pixel manipulation in main memory by the cpu.
The whole reason for having pixmaps initially, is that BlitzMax originally was based on OpenGL only and the OpenGL specification requires that the textures be in video ram in order to use them. Since it has features then to move graphics data to and from main memory, it refers to images in main memory as pixmaps (pixel maps). Textures are uploaded from pixmaps to textures and vice versa can be downloaded to a pixmap or the pixmap can be drawn directly to the backbuffer with glDrawPixels(). I think BRL also has other uses for pixmaps such as needing them in order to load in images from disk and for some pixel manipulation in main memory by the cpu.