Reset texture data to 0 after each 16 iterations to avoid overflow

Reset texture data to 0 after each 16 iterations to avoid overflow

The default precision of int in the fragment shader is mediump. The fs_out_color
in GeometryShaderMaxCombinedTextureUnitsTest::m_fragment_shader_code_body will
overflow when the number of texture unit become large, since each texture data
is equal to index of the texture units.
So reset texture data to 0 after each 16 iterations to avoid the overflow.

Component: OpenGL

Affected tests: KHR-GLES31.core.geometry_shader.limits.max_combined_texture_units

VK-GL-CTS issue: 2622

Change-Id: Icc9f2ed77a5a1cf6f315605f5f1045a60a56396c
(cherry picked from commit ccb924af7b96ee3771ce25f6d31a0b243bbddee1)