Fix OIT on Mac with explicit frag output binding (#332)

Author: jamesgreen26

common/src/backend/java/dev/engine_room/flywheel/backend/compile/PipelineCompiler.java

@@ -181,6 +181,20 @@ static PipelineCompiler create(ShaderSources sources, Pipeline pipeline, List<So
 					program.bindAttribLocation("_flw_aOverlay", 3);
 					program.bindAttribLocation("_flw_aLight", 4);
 					program.bindAttribLocation("_flw_aNormal", 5);
+
+					// Explicitly bind the fragment outputs to the color attachment slots the
+					// OitFramebuffer draw buffers expect. Each OIT pass only declares one of these
+					// output groups (guarded by #ifdefs), and binding a name that isn't present is a
+					// no-op, so binding them all unconditionally is safe. Apple's GL driver assigns
+					// implicit output locations in an order that doesn't match declaration order,
+					// which scrambles the coefficient targets and makes OIT geometry invisible.
+					program.bindFragDataLocation("_flw_outputColor", 0);
+					program.bindFragDataLocation("_flw_depthRange_out", 0);
+					program.bindFragDataLocation("_flw_coeffs0", 0);
+					program.bindFragDataLocation("_flw_coeffs1", 1);
+					program.bindFragDataLocation("_flw_coeffs2", 2);
+					program.bindFragDataLocation("_flw_coeffs3", 3);
+					program.bindFragDataLocation("_flw_accumulate", 0);
 				})
 				.postLink((key, program) -> {
 					Uniforms.setUniformBlockBindings(program);

common/src/backend/java/dev/engine_room/flywheel/backend/gl/shader/GlProgram.java

@@ -10,6 +10,7 @@
 import static org.lwjgl.opengl.GL20.glUniform4f;
 import static org.lwjgl.opengl.GL20.glUniformMatrix3fv;
 import static org.lwjgl.opengl.GL20.glUniformMatrix4fv;
+import static org.lwjgl.opengl.GL30.glBindFragDataLocation;
 import static org.lwjgl.opengl.GL30.glUniform1ui;
 import static org.lwjgl.opengl.GL30.glUniform2ui;
 import static org.lwjgl.opengl.GL31.GL_INVALID_INDEX;
@@ -189,6 +190,22 @@ public void bindAttribLocation(String attribute, int binding) {
 		glBindAttribLocation(handle(), binding, attribute);
 	}
 
+	/**
+	 * Binds a fragment shader output variable to a specific color attachment (draw buffer) slot.
+	 *
+	 * <p>This must happen before the program is linked. Binding a name that isn't an active output
+	 * in the linked program is a no-op, so it's safe to bind every possible output unconditionally.
+	 *
+	 * <p>Without this, the GL implementation is free to assign output locations in any order it
+	 * likes when a fragment shader declares multiple outputs without explicit {@code layout(location)}
+	 * qualifiers. Most desktop drivers happen to assign them in declaration order, but Apple's legacy
+	 * OpenGL driver does not, which scrambles the OIT coefficient targets and makes the transparent
+	 * geometry vanish. Binding the locations explicitly is correct everywhere.
+	 */
+	public void bindFragDataLocation(String name, int binding) {
+		glBindFragDataLocation(handle(), binding, name);
+	}
+
 	@Override
 	protected void deleteInternal(int handle) {
 		glDeleteProgram(handle);