Added a demonstration that using a grayscale image as alphachannel texture.

In this case, the grayscale image is used as a template. User can
choose arbitary color to fill into black area of the grayscale
image. White area is treated as tranparent.

Technique demonstrated here can also be applied to font rendering with
Freetype2. In Freetype2, a grayscale image is generated for each
selected character (glyph). Those grayscale images can be rendered
with method used in this code (gl_texture_grayscale.c).
Texture data will be freed after texture setup is done in the graphics card.

After calling glTexImage2D, texture image data is uploaded into
graphics card memory. The copy in the main memory thus can (should) be
recycled for other uses.