Screen for 32-Bit Octave on Linux rebuilt for testing.
Fix BlurredMipmapDemoShader.frag.txt for BlurredMipMapDemo.

was missing, causing this to fail on some GPU/Driver combos.
Apparently the original test system was lenient wrt this.

Use of texture2DLod() in fragment shaders like this one
either requires the above extension, or #version 130 for
GLSL version 1.3.