[WebGL] blitOffscreenFramebuffer: handle already-deleted program (#23980)

See comment for explanation.

Issue: #23654
See the other PR for additional discussion:
#23658 (comment)

Author: kainino0x

src/lib/libwebgl.js

@@ -993,6 +993,11 @@ for (/**@suppress{duplicate}*/var i = 0; i <= {{{ GL_POOL_TEMP_BUFFERS_SIZE }}};
 
         var prevProgram = gl.getParameter(0x8B8D /*GL_CURRENT_PROGRAM*/);
         gl.useProgram(context.blitProgram);
+        // If prevProgram was already marked for deletion, then, since it was
+        // still bound, it was not *actually* deleted. Binding a new program
+        // just now, thus, deleted the old one. This makes it impossible to
+        // restore. Hopefully the application didn't actually need it!
+        if (!gl.isProgram(prevProgram)) prevProgram = null;
 
         var prevVB = gl.getParameter(0x8894 /*GL_ARRAY_BUFFER_BINDING*/);
         gl.bindBuffer(0x8892 /*GL_ARRAY_BUFFER*/, context.blitVB);

test/browser/webgl_draw_triangle.c

@@ -5,6 +5,7 @@
  * found in the LICENSE file.
  */
 
+#include <assert.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <emscripten/emscripten.h>
@@ -104,8 +105,13 @@ int main() {
   glDrawArrays(GL_TRIANGLES, 0, 3);
 
 #ifdef EXPLICIT_SWAP
+  // Mark the program for deletion, first, as a regression test for the state
+  // restoration. https://github.com/emscripten-core/emscripten/issues/23654
+  glDeleteProgram(program);
+
   emscripten_webgl_commit_frame();
 #endif
 
+  assert(glGetError() == GL_NO_ERROR);
   return 0;
 }