[client] egl: switch to using GL_TEXTURE_EXTERNAL_OES for DMABUF

According to Erik @ NVidia the open source NVidia driver will not
create a EGLImage from a DMABUF if the target is not
GL_TEXTURE_EXTERNAL_OES. This change set converts the dmabuf texture
from GL_TEXTURE_2D to GL_TEXTURE_EXTERNAL_OES and at runtime performs a
global search & replace on fragment shaders as needed to remain
compatible, replacing `sampler2D` with `samplerExternalOES`.

Ref: https://github.com/NVIDIA/open-gpu-kernel-modules/discussions/243#discussioncomment-3283415

client/renderers/EGL/framebuffer.c

@@ -100,9 +100,7 @@ void egl_framebufferBind(EGL_Framebuffer * this)
   glViewport(0, 0, this->tex->format.width, this->tex->format.height);
 }
 
-GLuint egl_framebufferGetTexture(EGL_Framebuffer * this)
+EGL_Texture * egl_framebufferGetTexture(EGL_Framebuffer * this)
 {
-  GLuint output;
-  egl_textureGet(this->tex, &output, NULL, NULL);
-  return output;
+  return this->tex;
 }