URenderPipeline: Fix blend state with default/mc shaders on 1.17-1.20.6

On these versions, the blend state configured for the URenderPipeline
would under specific circumstances (depending on bugs in MC's
GlBlendState and previous setup of global state) not be honored.

See code comment for how this commit fixes / works around this issue.

GitHub: #117

Author: Johni0702

src/main/kotlin/gg/essential/universal/render/URenderPipeline.kt

@@ -112,6 +112,29 @@ class URenderPipeline private constructor(
 
     internal fun bind() {
         shader?.bind(glState.blendState)
+
+        // These seemingly pointless calls are used to bypass the builtin `GlBlendState` applied by Minecraft's
+        // `ShaderProgram` class right before the actual draw call (so no way for us to change it afterwards) on these
+        // versions.
+        // This is so we can apply the BlendState of this URenderPipeline, even when using a pre-existing ShaderProgram
+        // (such as all the default programs) which has a different builtin `GlBlendState`.
+        // This works by exploiting one of the (many) bugs of `GlBlendState`, namely that it is lazy in that it does
+        // not update anything if the previously active blend state matches the new one. In general that behavior is
+        // wrong because there's plenty other things which update the global GL state but we use this to our advantage
+        // here: We'll apply its GlBlendState now, and then revert the global GL state to our expected state, so that
+        // later when MC calls `ShaderProgram.bind` right before the actual draw, its `GlBlendState.enable` call will
+        // no-op and our desired blend state will stay active.
+        // (Prior to 1.17, we only support the fixed-function pipeline and our own shader class which we call with
+        //  `skipBlendState = true`, so it doesn't mess with the global blend state at all)
+        // (On 1.21+, MC's `GlBlendState` is still broken, but `ShaderProgram` no longer applies it)
+        //#if MC>=11700 && MC<12100
+        //$$ RenderSystem.getShader()?.let { shaderProgram ->
+        //$$     val prevBlendState = BlendState.active()
+        //$$     shaderProgram.bind()
+        //$$     shaderProgram.unbind()
+        //$$     UGraphics.Globals.blendState(prevBlendState)
+        //$$ }
+        //#endif
     }
 
     internal fun unbind() {