glgsg: fix texture extraction of MRTs in OpenGL ES

Fixes #815

panda/src/glstuff/glGraphicsStateGuardian_src.cxx

@@ -2523,6 +2523,20 @@ reset() {
   }
 #endif
 
+#if defined(OPENGLES) && !defined(OPENGLES_1)
+  if (is_at_least_gles_version(3, 0)) {
+    _glReadBuffer = (PFNGLREADBUFFERPROC)
+      get_extension_func("glReadBuffer");
+
+  } else if (has_extension("GL_NV_read_buffer")) {
+    _glReadBuffer = (PFNGLREADBUFFERPROC)
+      get_extension_func("glReadBufferNV");
+
+  } else {
+    _glReadBuffer = nullptr;
+  }
+#endif
+
 #ifndef OPENGLES_1
   _max_color_targets = 1;
   if (_glDrawBuffers != nullptr) {
@@ -8868,7 +8882,7 @@ set_draw_buffer(int rbtype) {
  */
 void CLP(GraphicsStateGuardian)::
 set_read_buffer(int rbtype) {
-#ifndef OPENGLES  // Draw buffers not supported by OpenGL ES. (TODO!)
+#ifndef OPENGLES_1  // Draw buffers not supported by OpenGL ES 1.
   if (rbtype & (RenderBuffer::T_depth | RenderBuffer::T_stencil)) {
     // Special case: don't have to call ReadBuffer for these.
     return;
@@ -8901,10 +8915,14 @@ set_read_buffer(int rbtype) {
       }
       ++index;
     }
+#ifdef OPENGLES
+    _glReadBuffer(buffer);
+#else
     glReadBuffer(buffer);
+#endif
 
   } else {
-
+#ifndef OPENGLES
     switch (rbtype & RenderBuffer::T_color) {
     case RenderBuffer::T_front:
       glReadBuffer(GL_FRONT);
@@ -8941,10 +8959,11 @@ set_read_buffer(int rbtype) {
     default:
       break;
     }
+#endif  // OPENGLES
   }
 
   report_my_gl_errors();
-#endif  // OPENGLES
+#endif  // OPENGLES_1
 }
 
 /**

panda/src/glstuff/glGraphicsStateGuardian_src.h

@@ -92,6 +92,7 @@ typedef void (APIENTRYP PFNGLGENBUFFERSPROC) (GLsizei n, GLuint *buffers);
 typedef void (APIENTRYP PFNGLBINDBUFFERPROC) (GLenum target, GLuint buffer);
 typedef void (APIENTRYP PFNGLBUFFERSUBDATAPROC) (GLenum target, GLintptr offset, GLsizeiptr size, const GLvoid *data);
 typedef void (APIENTRYP PFNGLDRAWBUFFERSPROC) (GLsizei n, const GLenum *bufs);
+typedef void (APIENTRYP PFNGLREADBUFFERPROC) (GLenum src);
 typedef void (APIENTRYP PFNGLCLEARBUFFERFVPROC) (GLenum buffer, GLint drawbuffer, const GLfloat *value);
 typedef void (APIENTRYP PFNGLBUFFERDATAPROC) (GLenum target, GLsizeiptr size, const GLvoid *data, GLenum usage);
 typedef void (APIENTRYP PFNGLDELETEBUFFERSPROC) (GLsizei n, const GLuint *buffers);
@@ -935,6 +936,10 @@ public:
   PFNGLBLITFRAMEBUFFEREXTPROC _glBlitFramebuffer;
   PFNGLDRAWBUFFERSPROC _glDrawBuffers;
 
+#if defined(OPENGLES) && !defined(OPENGLES_1)
+  PFNGLREADBUFFERPROC _glReadBuffer;
+#endif
+
 #ifndef OPENGLES_1
   PFNGLCLEARBUFFERFVPROC _glClearBufferfv;
   PFNGLCLEARBUFFERIVPROC _glClearBufferiv;