gpu: implement automatic mipmaps for images

All GPU APIs except OpenGL ES 2 can generate mipmaps for textures.
This trades 33% more GPU memory use for improved rendering quality
and speed for downscaled images.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

internal/gl/gl_js.go

@@ -72,6 +72,7 @@ type Functions struct {
 	_flush                             js.Value
 	_framebufferRenderbuffer           js.Value
 	_framebufferTexture2D              js.Value
+	_generateMipmap                    js.Value
 	_getRenderbufferParameteri         js.Value
 	_getFramebufferAttachmentParameter js.Value
 	_getParameter                      js.Value
@@ -167,6 +168,7 @@ func NewFunctions(ctx Context, forceES bool) (*Functions, error) {
 		_flush:                             _bind(webgl, `flush`),
 		_framebufferRenderbuffer:           _bind(webgl, `framebufferRenderbuffer`),
 		_framebufferTexture2D:              _bind(webgl, `framebufferTexture2D`),
+		_generateMipmap:                    _bind(webgl, `generateMipmap`),
 		_getRenderbufferParameteri:         _bind(webgl, `getRenderbufferParameteri`),
 		_getFramebufferAttachmentParameter: _bind(webgl, `getFramebufferAttachmentParameter`),
 		_getParameter:                      _bind(webgl, `getParameter`),
@@ -419,6 +421,9 @@ func (f *Functions) FramebufferRenderbuffer(target, attachment, renderbuffertarg
 func (f *Functions) FramebufferTexture2D(target, attachment, texTarget Enum, t Texture, level int) {
 	f._framebufferTexture2D.Invoke(int(target), int(attachment), int(texTarget), js.Value(t), level)
 }
+func (f *Functions) GenerateMipmap(target Enum) {
+	f._generateMipmap.Invoke(int(target))
+}
 func (f *Functions) GetError() Enum {
 	// Avoid slow getError calls. See gio#179.
 	return 0