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

There another bug in GlBlendState which cause the GlBlendState which we
apply by enabling the shader to not actually be stored as the
`activeBlendState`.
See code comments for details.

Since we now necessarily have access to the GlBlendState of the shader,
this commit also simplifies our dummy call to merely enable that
particular GlBlendState instead of binding and unbinding the entire
shader.

Linear: EM-3512
GitHub: #121

Author: Johni0702

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

@@ -51,6 +51,10 @@ import net.minecraft.client.renderer.WorldVertexBufferUploader
 //$$ import net.minecraft.client.render.Shader
 //$$ import java.util.function.Supplier
 //#endif
+//#if MC<12100
+//$$ import net.minecraft.client.gl.GlBlendState
+//$$ import java.lang.invoke.MethodHandles
+//#endif
 //#else
 import gg.essential.universal.shader.GlShader
 import net.minecraft.client.renderer.vertex.VertexFormatElement
@@ -129,9 +133,23 @@ class URenderPipeline private constructor(
         // (On 1.21+, MC's `GlBlendState` is still broken, but `ShaderProgram` no longer applies it)
         //#if MC>=11700 && MC<12100
         //$$ RenderSystem.getShader()?.let { shaderProgram ->
+        //$$     // Capture actual desired blend state to restore afterwards
         //$$     val prevBlendState = BlendState.active()
-        //$$     shaderProgram.bind()
-        //$$     shaderProgram.unbind()
+        //$$     // This seemingly pointless change of active blend state here is necessary because of another bug in
+        //$$     // GlBlendState: It'll only update its global `activeBlendState` when the blend state is enabled or disabled
+        //$$     // but not when merely the blend function or one of its parameters is changed. So we need to make sure
+        //$$     // we enable a blend state which is the opposite of the blend state of the shader before we enable that one.
+        //$$     // We need to do two calls because we don't know the state of the current `activeBlendState`.
+        //$$     if (shaderProgram.glBlendState.isBlendDisabled) {
+        //$$         BlendState.DISABLED.mc.enable()
+        //$$         BlendState.ALPHA.mc.enable()
+        //$$     } else {
+        //$$         BlendState.ALPHA.mc.enable()
+        //$$         BlendState.DISABLED.mc.enable()
+        //$$     }
+        //$$     // Enable shader's GlBlendState so it becomes the `GlBlendState.activeBlendState`
+        //$$     shaderProgram.glBlendState.enable()
+        //$$     // Restore desired blend state
         //$$     prevBlendState.activateWithoutChangingMcBlendState()
         //$$ }
         //#endif
@@ -522,6 +540,22 @@ class URenderPipeline private constructor(
         val isRequired = false
         //#endif
 
+        //#if MC>=11700 && MC<12100
+        //$$ private val Shader_glBlendState: (Shader) -> GlBlendState by lazy {
+        //$$     try {
+        //$$         val field = Shader::class.java.declaredFields
+        //$$             .first { it.type == GlBlendState::class.java }
+        //$$         field.isAccessible = true
+        //$$         val handle = MethodHandles.lookup().unreflectGetter(field)
+        //$$         return@lazy { shader: Shader -> handle.invoke(shader) as GlBlendState }
+        //$$     } catch (e: Exception) {
+        //$$         e.printStackTrace()
+        //$$         return@lazy { GlBlendState() }
+        //$$     }
+        //$$ }
+        //$$ private val Shader.glBlendState: GlBlendState get() = Shader_glBlendState(this)
+        //#endif
+
         //#if !STANDALONE
         //#if MC>=12105
         //$$ @JvmStatic