[client] egl: implement partial copies for framebuffer textures

This uses the same line sweep algorithm originally created to copy DXGI
textures to IVSHMEM to implement the copy from IVSHMEM to memory-mapped
pixel buffer objects.

client/renderers/EGL/texture_buffer.h

@@ -24,6 +24,8 @@
 #include "texture_util.h"
 #include "common/locking.h"
 
+#define EGL_TEX_BUFFER_MAX 2
+
 typedef struct TextureBuffer
 {
   EGL_Texture base;
@@ -31,9 +33,9 @@ typedef struct TextureBuffer
 
   EGL_TexFormat format;
   int           texCount;
-  GLuint        tex[2];
+  GLuint        tex[EGL_TEX_BUFFER_MAX];
   GLuint        sampler;
-  EGL_TexBuffer buf[2];
+  EGL_TexBuffer buf[EGL_TEX_BUFFER_MAX];
   int           bufFree;
   GLsync        sync;
   LG_Lock       copyLock;