Enable trilinear filtering for OpenGL

Improve downscaling quality if mipmapping is available.

Suggested-by: Giumo Clanjor (哆啦比猫/兰威举) <cjxgm2@gmail.com>

Fixes #40 <https://github.com/Genymobile/scrcpy/issues/40>
Ref: <https://github.com/Genymobile/scrcpy/issues/40#issuecomment-591917787>

app/src/opengl.h

@@ -0,0 +1,36 @@
+#ifndef SC_OPENGL_H
+#define SC_OPENGL_H
+
+#include <stdbool.h>
+#include <SDL2/SDL_opengl.h>
+
+#include "config.h"
+
+struct sc_opengl {
+    const char *version;
+    bool is_opengles;
+    int version_major;
+    int version_minor;
+
+    const GLubyte *
+    (*GetString)(GLenum name);
+
+    void
+    (*TexParameterf)(GLenum target, GLenum pname, GLfloat param);
+
+    void
+    (*TexParameteri)(GLenum target, GLenum pname, GLint param);
+
+    void
+    (*GenerateMipmap)(GLenum target);
+};
+
+void
+sc_opengl_init(struct sc_opengl *gl);
+
+bool
+sc_opengl_version_at_least(struct sc_opengl *gl,
+                           int minver_major, int minver_minor,
+                           int minver_es_major, int minver_es_minor);
+
+#endif