WebGL: Forbid GenerateMipmap on zero-size textures

src/libANGLE/ErrorStrings.h

@@ -88,6 +88,7 @@ MSG kFramebufferTextureInvalidMipLevel = "Mip level invalid for framebuffer text
 MSG kFramebufferTextureLayerIncorrectTextureType = "Texture is not a three-dimensional or two-dimensionsal array texture.";
 MSG kGLES1Only = "GLES1-only function.";
 MSG kGenerateMipmapNotAllowed = "Texture format does not support mipmap generation.";
+MSG kGenerateMipmapZeroSize = "Cannot generate mipmaps for a zero-size texture in a WebGL context.";
 MSG kGeometryShaderExtensionNotEnabled = "GL_EXT_geometry_shader extension not enabled.";
 MSG kImmutableTextureBound = "The value of TEXTURE_IMMUTABLE_FORMAT for the texture currently bound to target on the active texture unit is true.";
 MSG kIncompatibleDrawModeAgainstGeometryShader = "Primitive mode is incompatible with the input primitive type of the geometry shader.";

src/libANGLE/validationES2.cpp

@@ -6201,6 +6201,14 @@ bool ValidateGenerateMipmap(Context *context, TextureType target)
         return false;
     }
 
+    if (context->getExtensions().webglCompatibility &&
+        (texture->getWidth(baseTarget, effectiveBaseLevel) == 0 ||
+         texture->getHeight(baseTarget, effectiveBaseLevel) == 0))
+    {
+        context->validationError(GL_INVALID_OPERATION, kGenerateMipmapZeroSize);
+        return false;
+    }
+
     return true;
 }