Ported graphics/GlTexture from sdl2-gamedev

Ported graphics/GlTexture from sdl2-gamedev
    src/graphics/GlTexture.cpp (+)
    src/graphics/GlTexture.h   (+)
Ported util/TestTextBoxText from sdl2-gamedev

Ported util/TestTextBoxText from sdl2-gamedev
    src/util/TestTextBoxText.h   (+)
Added utility for generating test coverage reports

Added utility for generating test coverage reports using gcov and lcov

The report is created using the added ./gencoverage.sh utility, creates an html
page using lcov containing an easy to understand line coverage report.

    README.md
    common_build.py
    devtools.sh
    gencoverage.sh   (+)
    src/SConscript
    src/SConscript_tests