Kinda works, but then not...

tr_backend.c

// When it works: https://i.gyazo.com/cf1ddce65f0eb0233e2389e63487a8ee.jpg
// When it doesn't work: https://i.gyazo.com/42e3ef72ffa244c26f2a9503e70af5e6.png
// I am calling simpletriangle() in renderer2/tr_backend.c at the end of RB_RenderViewFront:
// https://github.com/etlegacy/etlegacy/blob/master/src/renderer2/tr_backend.c#L6045

static void simpletriangle() {
	GLuint shader_programme;
	GLuint vao;

	//GLint oldid;
	//glGetIntegerv(GL_CURRENT_PROGRAM, &oldid);

	float points[] = {
	0.0f,  0.5f,  0.0f,
	0.5f, -0.5f,  0.0f,
	-0.5f, -0.5f,  0.0f
	};

	GLuint vbo = 0;
	glGenBuffers(1, &vbo);
	glBindBuffer(GL_ARRAY_BUFFER, vbo);
	glBufferData(GL_ARRAY_BUFFER, 9 * sizeof(float), points, GL_STATIC_DRAW);

	vao = 0;
	glGenVertexArrays(1, &vao);
	glBindVertexArray(vao);
	glEnableVertexAttribArray(0);
	glBindBuffer(GL_ARRAY_BUFFER, vbo);
	glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, NULL);

#if 0
	// This was just for testing, I'm using the Q3 Shader API now

	const char* vertex_shader =
	"#version 400\n"
	"in vec3 vp;"
	"void main() {"
	"  gl_Position = vec4(vp, 1.0);"
	"}";
	
	const char* fragment_shader =
	"#version 400\n"
	"out vec4 frag_colour;"
	"void main() {"
	"  frag_colour = vec4(0.5, 0.0, 0.5, 1.0);"
	"}";
	
	GLuint vs = glCreateShader(GL_VERTEX_SHADER);
	glShaderSource(vs, 1, &vertex_shader, NULL);
	glCompileShader(vs);
	GLuint fs = glCreateShader(GL_FRAGMENT_SHADER);
	glShaderSource(fs, 1, &fragment_shader, NULL);
	glCompileShader(fs);
	
	shader_programme = glCreateProgram();
	glAttachShader(shader_programme, fs);
	glAttachShader(shader_programme, vs);
	glLinkProgram(shader_programme);
	glDeleteShader(vs);
	glDeleteShader(fs);

	glUseProgram(shader_programme);
#else

/*
	This needs 1 changed file and 2 new files:

	Change file glsl/tr_glslsources.c line 3766
		"cameraEffects\n"
		"{\n"
		"\tfilename \"cameraEffects\"\n"
		"\tattribs ATTR_POSITION ATTR_TEXCOORD\n"
		"}\n"
		"kung_testshader\n"
		"{\n"
		"\tfilename \"kung_testshader\"\n"
		// I don't set attribs because my test shader doesn't have any
		//"\tattribs ATTR_POSITION ATTR_TEXCOORD\n"
		"}\n"

	Create file glsl/kung_testshader_vp.glsl:
		in vec3 vp;
		void main() {
			gl_Position = vec4(vp, 1.0);
		}

	Create file glsl/kung_testshader_fp.glsl:
		out vec4 frag_colour;
		void main() {
			frag_colour = vec4(0.5, 0.0, 0.5, 1.0);
		}
*/

	// Both fuck up the scene
	//SetMacrosAndSelectProgram(trProg.kung_testshader);
	SelectProgram(trProg.kung_testshader);
#endif

	//glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
	glBindVertexArray(vao);
	// draw points 0-3 from the currently bound VAO with current in-use shader
	glDrawArrays(GL_TRIANGLES, 0, 3);
	//glBindVertexArray(0);

	//glDeleteBuffers(1, &vbo);
	//glDeleteProgram(shader_programme);

	//glUseProgram(oldid);
}