glgsg: detect and fix overlapping SSBO bindings

Fixes #1176

panda/src/glstuff/glGraphicsStateGuardian_src.cxx

@@ -2134,6 +2134,8 @@ reset() {
        get_extension_func("glGetProgramResourceName");
     _glGetProgramResourceiv = (PFNGLGETPROGRAMRESOURCEIVPROC)
        get_extension_func("glGetProgramResourceiv");
+    _glShaderStorageBlockBinding = (PFNGLSHADERSTORAGEBLOCKBINDINGPROC)
+       get_extension_func("glShaderStorageBlockBinding");
   } else
 #endif
   {

panda/src/glstuff/glGraphicsStateGuardian_src.h

@@ -1068,6 +1068,7 @@ public:
   PFNGLGETPROGRAMINTERFACEIVPROC _glGetProgramInterfaceiv;
   PFNGLGETPROGRAMRESOURCENAMEPROC _glGetProgramResourceName;
   PFNGLGETPROGRAMRESOURCEIVPROC _glGetProgramResourceiv;
+  PFNGLSHADERSTORAGEBLOCKBINDINGPROC _glShaderStorageBlockBinding;
 #endif  // !OPENGLES
 
   GLenum _edge_clamp;

panda/src/glstuff/glShaderContext_src.cxx

@@ -341,6 +341,8 @@ CLP(ShaderContext)(CLP(GraphicsStateGuardian) *glgsg, Shader *s) : ShaderContext
     block_maxlength = max(64, block_maxlength);
     char *block_name_cstr = (char *)alloca(block_maxlength);
 
+    BitArray bindings;
+
     for (int i = 0; i < block_count; ++i) {
       block_name_cstr[0] = 0;
       _glgsg->_glGetProgramResourceName(_glsl_program, GL_SHADER_STORAGE_BLOCK, i, block_maxlength, nullptr, block_name_cstr);
@@ -349,6 +351,20 @@ CLP(ShaderContext)(CLP(GraphicsStateGuardian) *glgsg, Shader *s) : ShaderContext
       GLint values[2];
       _glgsg->_glGetProgramResourceiv(_glsl_program, GL_SHADER_STORAGE_BLOCK, i, 2, props, 2, nullptr, values);
 
+      if (bindings.get_bit(values[0])) {
+        // Binding index already in use, assign a different one.
+        values[0] = bindings.get_lowest_off_bit();
+        _glgsg->_glShaderStorageBlockBinding(_glsl_program, i, values[0]);
+      }
+      bindings.set_bit(values[0]);
+
+      if (GLCAT.is_debug()) {
+        GLCAT.debug()
+          << "Active shader storage block " << block_name_cstr
+          << " with size " << values[1] << " is bound to binding "
+          << values[0] << "\n";
+      }
+
       StorageBlock block;
       block._name = InternalName::make(block_name_cstr);
       block._binding_index = values[0];