Image_PixMap::ImgFormat - extend enumeration by ImgAlpha and ImgAlphaF.
OpenGl_Workspace::setTextureParams() - specify GL_REPLACE for 1-component textures with disabled modulation.
OpenGl_Texture::GetDataFormat() - return GL_LUMINANCE format for ImgGray format and GL_ALPHA for ImgAlpha.
vmarkerstest - override pixel format for grayscale images to ImgAlpha.
Add test case bugs/vis/bug25544_graytexture.