+ * Update the vertex/fragment program's TexturesUsed array.
+ *
+ * This needs to be called after glUniform(set sampler var) is called.
+ * A call to glUniform(samplerVar, value) causes a sampler to point to a
+ * particular texture unit. We know the sampler's texture target
+ * (1D/2D/3D/etc) from compile time but the sampler's texture unit is
+ * set by glUniform() calls.
+ *
+ * So, scan the program->SamplerUnits[] and program->SamplerTargets[]
+ * information to update the prog->TexturesUsed[] values.
+ * Each value of TexturesUsed[unit] is one of zero, TEXTURE_1D_INDEX,
+ * TEXTURE_2D_INDEX, TEXTURE_3D_INDEX, etc.
+ * We'll use that info for state validation before rendering.