Fix tex unit assignment to std::optional type

[heinezen]

Fixes a small bug in the storage of the texture to tex unit bindings in the shader program.

openage/libopenage/renderer/opengl/shader_program.cpp

Lines 427 to 429 in 02d3242

	auto &tex_value = *this->textures_per_texunits[tex_unit_id];
	tex_value = tex;
	break;

The current code (see above) assumes that there always exists a binding from tex unit to texture and the std::optional type always contains a value. However, this is not always the case, e.g. when the shader is first initialized and no textures have been bound yet. We also wouldn't need to use std::optional then, don't we?