Without this patch, imv fails to display images with width/height
larger than OpenGL's GL_MAX_TEXTURE_SIZE (usually these days: 16384),
as it tries to make a texture equal to the bitmap->width x bitmap->height.
This patch divides up the image into multiple adjoining textures, each of
whose dimensions are smaller than GL_MAX_TEXTURE_SIZE. To avoid 'slivers',
the width/height of the original image are divided into equal-sized pieces.