gpu,gpu/gl: introduce Backend

A recent change made the OpenGL functions an interface of the functions
required for the implementation of GPU, a renderer for Gio operations.
That allowed for running Gio on external systems where OpenGL is
available.

However, to allow for non-OpenGL flavored backends such as Vulkan,
Metal and Direct3D, this change introduces Backend for the high-level
operations required by GPU. This change also adds a concrete backend
to package gl.

Type Backend is a first cut heavily based on OpenGL. Future changes will add
more backends, where the Backend interface quite possibly will need refinement.

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

gpu/backend.go

@@ -0,0 +1,156 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package gpu
+
+import (
+	"image"
+	"time"
+)
+
+// Backend represents the abstraction of underlying GPU
+// APIs such as OpenGL, Direct3D useful for rendering Gio
+// operations.
+type Backend interface {
+	BeginFrame()
+	EndFrame()
+	Caps() Caps
+	NewTimer() Timer
+	// IsContinuousTime reports whether all timer measurements
+	// are valid at the point of call.
+	IsTimeContinuous() bool
+	NewTexture(minFilter, magFilter TextureFilter) Texture
+	DefaultFramebuffer() Framebuffer
+	NilTexture() Texture
+	NewFramebuffer() Framebuffer
+	NewBuffer(typ BufferType) Buffer
+	NewProgram(vertexShader, fragmentShader string, attribMap []string) (Program, error)
+	SetupVertexArray(slot int, size int, dataType DataType, stride, offset int)
+
+	DepthFunc(f DepthFunc)
+	ClearColor(r, g, b, a float32)
+	ClearDepth(d float32)
+	Clear(buffers BufferAttachments)
+	Viewport(x, y, width, height int)
+	DrawArrays(mode DrawMode, off, count int)
+	DrawElements(mode DrawMode, off, count int)
+	SetBlend(enable bool)
+	SetDepthTest(enable bool)
+	DepthMask(mask bool)
+	BlendFunc(sfactor, dfactor BlendFactor)
+}
+
+type BlendFactor uint8
+
+type DrawMode uint8
+
+type BufferAttachments uint
+
+type TextureFilter uint8
+type TextureFormat uint8
+
+type BufferType uint8
+type BufferUsage uint8
+
+type DataType uint8
+
+type DepthFunc uint8
+
+type Features uint
+
+type Caps struct {
+	Features       Features
+	MaxTextureSize int
+}
+
+type Program interface {
+	Bind()
+	Release()
+	UniformFor(uniform string) Uniform
+	Uniform1i(u Uniform, v int)
+	Uniform1f(u Uniform, v float32)
+	Uniform2f(u Uniform, v0, v1 float32)
+	Uniform4f(u Uniform, v0, v1, v2, v3 float32)
+}
+
+type Uniform interface{}
+
+type Buffer interface {
+	Bind()
+	Upload(usage BufferUsage, data []byte)
+	Release()
+}
+
+type Framebuffer interface {
+	Bind()
+	BindTexture(t Texture)
+	Invalidate()
+	Release()
+	IsComplete() error
+}
+
+type Timer interface {
+	Begin()
+	End()
+	Duration() (time.Duration, bool)
+	Release()
+}
+
+type Texture interface {
+	Upload(img *image.RGBA)
+	Release()
+	Bind(unit int)
+	Resize(format TextureFormat, width, height int)
+}
+
+const (
+	BufferAttachmentColor BufferAttachments = 1 << iota
+	BufferAttachmentDepth
+)
+
+const (
+	DepthFuncGreater DepthFunc = iota
+)
+
+const (
+	DataTypeFloat DataType = iota
+	DataTypeShort
+)
+
+const (
+	BufferUsageStaticDraw BufferUsage = iota
+)
+
+const (
+	BufferTypeIndices BufferType = iota
+	BufferTypeData
+)
+
+const (
+	TextureFormatSRGB TextureFormat = iota
+	TextureFormatFloat
+)
+
+const (
+	FilterNearest TextureFilter = iota
+	FilterLinear
+)
+
+const (
+	FeatureTimers Features = iota
+)
+
+const (
+	DrawModeTriangleStrip DrawMode = iota
+	DrawModeTriangles
+)
+
+const (
+	BlendFactorOne BlendFactor = iota
+	BlendFactorOneMinusSrcAlpha
+	BlendFactorZero
+	BlendFactorDstColor
+)
+
+func (f Features) Has(feats Features) bool {
+	return f&feats == feats
+}