progs/tests: added new blitfb.c test

Tests glBlitFramebuffer() between two texture/renderbuffer surfaces.
In particular, blit from level[1] of a cube map face to a 2D texture.

Used to find/fix bug in intel do_copy_texsubimage().
See commit aef1ab1073f3e30d699b99dae17518ed48b57c72

progs/tests/SConscript

@@ -52,6 +52,7 @@ progs = [
     'blendminmax',
     'blendsquare',
     'blendxor',
+    'blitfb',
     'bufferobj',
     'bug_3050',
     'bug_3101',