[WebGL] blitOffscreenFramebuffer: handle already-deleted program

See comment for explanation.

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

Author: kainino0x

src/lib/libwebgl.js

@@ -994,6 +994,10 @@ 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. Unbinding it deletes it.
+        // This makes it impossible to restore. Hope the app didn't 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);