Add test for glTexImage2D for GL_LUMINANCE and GL_FLOAT. #3039.
Add support for glTexImage2D and GL_LUMINANCE + GL_FLOAT (which require OES_texture_float extension)
Add support for GL_LUMINANCE and GL_HALF_FLOAT textures in glTexImage2D (requires OES_texture_half_float extensions)
Simplify GL getTexPixelData, and reuse that function in glReadPixels to avoid code duplication.