Fix libretro shaders on Windows

Force a binding value for sampler2D uniforms. glGetUniformiv() always
returns -1 for sampler2D with Windows AMD drivers, whereas Mesa returns
the correct binding value set in the GLSL source. Don't know for other
drivers, but forcing an arbitrary value is fine since sampler2D uniforms
are binded using built-in identifiers and texture names.

src/gui/gl_shader_program.cpp

@@ -47,6 +47,9 @@ GLShaderProgram::Uniform::Uniform(GLuint _program, GLuint _index, GLint _binding
 		case GL_UNSIGNED_INT_SAMPLER_2D:
 			if(_binding < 0) {
 				GLCALL( glGetUniformiv(program, index, &binding) );
+				if(binding < 0) {
+					PWARNF(LOG_V0, LOG_OGL, "The returned binding value for sampler '%s' is %d.\n", name.c_str(), binding);
+				}
 			} else {
 				binding = _binding;
 			}
@@ -245,13 +248,10 @@ GLShaderProgram::GLShaderProgram(std::vector<std::string> _vs_paths, std::vector
 	GLCALL( glGetProgramiv(m_gl_name, GL_ACTIVE_UNIFORMS, &active_uniforms) );
 
 	PINFOF(LOG_V2, LOG_OGL, " uniforms: %d\n", active_uniforms);
-	GLint ubinding = -1;
-	if(m_version < 420) {
-		ubinding = 0;
-	}
+	GLint ubinding = 0;
 	for(GLuint uidx=0; uidx<GLuint(active_uniforms); uidx++) {
 		auto & uni = m_uniforms.emplace_back(m_gl_name, uidx, ubinding);
-		if(m_version < 420 && uni.is_sampler()) {
+		if(uni.is_sampler()) {
 			ubinding = uni.binding + 1;
 		}
 		auto stduni = ms_builtin_uniform_names.find(uni.member_name);