ui/app/internal/gpu: introduce caps and context

Enables graceful fallback to OpenGL ES 2 and WebGL.

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

ui/app/internal/gpu/context.go

@@ -0,0 +1,52 @@
+package gpu
+
+import (
+	"strings"
+
+	"gioui.org/ui/app/internal/gl"
+)
+
+type context struct {
+	caps caps
+	*gl.Functions
+}
+
+type caps struct {
+	EXT_disjoint_timer_query bool
+	srgbMode                 srgbMode
+}
+
+type srgbMode uint8
+
+const (
+	srgbES3 srgbMode = iota
+	srgbEXT
+)
+
+func newContext(glctx gl.Context) (*context, error) {
+	ctx := &context{
+		Functions: glctx.Functions(),
+	}
+	exts := ctx.GetString(gl.EXTENSIONS)
+	glVer := ctx.GetString(gl.VERSION)
+	ver, err := gl.ParseGLVersion(glVer)
+	if err != nil {
+		return nil, err
+	}
+	ctx.caps = caps{
+		EXT_disjoint_timer_query: strings.Contains(exts, "GL_EXT_disjoint_timer_query"),
+		srgbMode:                 srgbModeFor(ver, exts),
+	}
+	return ctx, nil
+}
+
+func srgbModeFor(ver [2]int, exts string) srgbMode {
+	switch {
+	case ver[0] >= 3:
+		return srgbES3
+	case strings.Contains(exts, "EXT_sRGB"):
+		return srgbEXT
+	default:
+		panic("neither OpenGL ES 3 nor EXT_sRGB is supported")
+	}
+}