Add vendor

Specifically because I want to modify a dependency

vendor/golang.org/x/mobile/gl/doc.go

@@ -0,0 +1,66 @@
+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+/*
+Package gl implements Go bindings for OpenGL ES 2.0 and ES 3.0.
+
+The GL functions are defined on a Context object that is responsible for
+tracking a GL context. Typically a windowing system package (such as
+golang.org/x/exp/shiny/screen) will call NewContext and provide
+a gl.Context for a user application.
+
+If the gl package is compiled on a platform capable of supporting ES 3.0,
+the gl.Context object also implements gl.Context3.
+
+The bindings are deliberately minimal, staying as close the C API as
+possible. The semantics of each function maps onto functions
+described in the Khronos documentation:
+
+https://www.khronos.org/opengles/sdk/docs/man/
+
+One notable departure from the C API is the introduction of types
+to represent common uses of GLint: Texture, Surface, Buffer, etc.
+
+# Debug Logging
+
+A tracing version of the OpenGL bindings is behind the `gldebug` build
+tag. It acts as a simplified version of apitrace. Build your Go binary
+with
+
+	-tags gldebug
+
+and each call to a GL function will log its input, output, and any
+error messages. For example,
+
+	I/GoLog   (27668): gl.GenBuffers(1) [Buffer(70001)]
+	I/GoLog   (27668): gl.BindBuffer(ARRAY_BUFFER, Buffer(70001))
+	I/GoLog   (27668): gl.BufferData(ARRAY_BUFFER, 36, len(36), STATIC_DRAW)
+	I/GoLog   (27668): gl.BindBuffer(ARRAY_BUFFER, Buffer(70001))
+	I/GoLog   (27668): gl.VertexAttribPointer(Attrib(0), 6, FLOAT, false, 0, 0)  error: [INVALID_VALUE]
+
+The gldebug tracing has very high overhead, so make sure to remove
+the build tag before deploying any binaries.
+*/
+package gl // import "golang.org/x/mobile/gl"
+
+/*
+Implementation details.
+
+All GL function calls fill out a C.struct_fnargs and drop it on the work
+queue. The Start function drains the work queue and hands over a batch
+of calls to C.process which runs them. This allows multiple GL calls to
+be executed in a single cgo call.
+
+A GL call is marked as blocking if it returns a value, or if it takes a
+Go pointer. In this case the call will not return until C.process sends a
+value on the retvalue channel.
+
+This implementation ensures any goroutine can make GL calls, but it does
+not make the GL interface safe for simultaneous use by multiple goroutines.
+For the purpose of analyzing this code for race conditions, picture two
+separate goroutines: one blocked on gl.Start, and another making calls to
+the gl package exported functions.
+*/
+
+//go:generate go run gendebug.go -o gldebug.go